This a very old story. it happened few months ago.once i was talking with my friends .. we were talking about something punishment in hell.then while talking i joked that when we will be punished hell .we will also beat and my friends laughed including me and i dont remember exactly but i think once i also made fun of jinns at the same time..so i asked to well known saudi scholar that we joked about punishment in hell .so he told that this invalidiate your and your friends islam.i was shocked that how could this be.but now many months have passed and my friends have forgotten this.but after that they all performed umrah and also went to madinah.but they r not aware that we did this sin.so do they have to say shahada again?because one saudi scholar told me they dont have to shahada because u people keep saying shahada all the time. but i am afraid what if my frinds wouldnt said shahda since that time.so if i will remind them on phone about this incident.they will think i am mad and they will be angry.trust me this is not in my capacity to tell.i hope allah would have forgiven them.or should i still remind them that ur umrah,salaah etc nothing was accepted and now u have to say shahada again? because i came to know about nullifying islam recently and this thing happened many months before

Praise
be to Allah.

Firstly: 

Undoubtedly mocking and making fun of something that is known
to be part of Islam constitutes kufr (disbelief) that puts one beyond the
pale of Islam, even if the one who does that does not mean anything more
than joking, because  this in and of itself is an action that puts one
beyond the pale of Islam as Allah, may He be exalted, says (interpretation
of the meaning):


“If you ask them (about this), they declare: ‘We were only
talking idly and joking.’ Say: ‘Was it at Allah, and His Ayaat (proofs,
evidences, verses, lessons, signs, revelations, etc.) and His Messenger
(blessings and peace of Allah be upon him) that you were mocking?


Make no excuse; you have disbelieved after you had believed.
If We pardon some of you, We will punish others amongst you because they
were Mujrimoon (disbelievers, polytheists, sinners, criminals, etc.)”


[at-Tawbah 9:65-66]

Please see the answers to questions no.
175838
and 111473 

What the one who is present in such a gathering must do is
denounce that. If he cannot do that, then he must leave the gathering, so
that he will not be a partner in the sin. If he expresses any kind of
approval of their actions and words, then he is like them. Allah, may He be
exalted, says (interpretation of the meaning):


“And when you
(Muhammad (blessings and peace of Allah be upon him)) see those who engage
in a false conversation about Our Verses (of the Quran) by mocking at them,
stay away from them till they turn to another topic. And if Shaitan (Satan)
causes you to forget, then after the remembrance sit not you in the company
of those people who are the Zalimoon (polytheists and wrongdoers, etc.)”


[al-An‘aam 6:68]


“And it has
already been revealed to you in the Book (this Quran) that when you hear the
Verses of Allah being denied and mocked at, then sit not with them, until
they engage in a talk other than that; (but if you stayed with them)
certainly in that case you would be like them. Surely, Allah will collect
the hypocrites and disbelievers all together in Hell”


[an-Nisa’ 4:140]

Allah, may He be glorified and exalted, regards the one who
sits in a gathering and witnesses evil, if he does not denounce it, as being
like the one who does or speaks that evil. 

Hence it is said that the one who listens is the partner of
the one who backbites. According to a report, the one who witnesses sin but
hates it is like the one who was absent from it, and the one who was absent
from it but approves of it is like the one who witnessed it. 

Shaykh al-Islam ibn Taymiyah said:

When some of them made fun of (the believers) and said
concerning those with whom Allah is pleased, “We have never seen anyone like
our people who are well-versed in the Qur’an when it comes to eating more or
telling more lies or being more cowardly when meeting the enemy,” Allah
revealed the words (interpretation of the meaning):


“The hypocrites fear lest a Soorah (chapter of the Quran)
should be revealed about them, showing them what is in their hearts. Say:
“(Go ahead and) mock! But certainly Allah will bring to light all that you
fear.’


If you ask them (about this), they declare: ‘We were only
talking idly and joking.’ Say: ‘Was it at Allah, and His Ayaat (proofs,
evidences, verses, lessons, signs, revelations, etc.) and His Messenger
(blessings and peace of Allah be upon him) that you were mocking?


Make no excuse; you have disbelieved after you had believed.
If We pardon some of you, We will punish others amongst you because they
were Mujrimoon (disbelievers, polytheists, sinners, criminals, etc.)”


[al-Tawbah 9:66]
.

Allah informed them that they had disbelieved after having
believed, even though they said, “We say words of disbelief without
believing in them; rather we were just talking idly and joking.” He
explained that mocking the revelations of Allah constitutes kufr, and such
words cannot be uttered except by someone who is content with such words,
for if he had faith in his heart, it would have prevented him from saying
such words.

End quote from al-Eemaan (2/284) 

Secondly: 

Undoubtedly making fun of any aspect of Islam is contrary to
respect and veneration for this religion, especially with regard to matters
such as the punishment on the Day of Resurrection, or in the grave, or the
terrors of the resurrection. Islam has only told us of these things so as to
alarm people and set their hearts straight, so that they will not persist in
heedlessness. One of the greatest forms of heedlessness, which is indicative
of being overwhelmed by ignorance, whims and desires, and lack of insight,
is when one regards matters of exhortation and admonition as a joke and
something to be laughed at and made fun of. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said concerning these hypocrites who make fun of Islam: 

This indicates that they did not themselves think that they
had committed an act of kufr; rather they thought that that did not
constitute kufr. So Allah explained that ridiculing Allah and His
revelations and His Messenger constituted kufr and meant that the person had
disbelieved after having believed. This indicates that they had faith,
albeit weak, and they did this haraam action knowing that it was haraam, but
they did not think that it constituted kufr. However it was kufr by means of
which they became disbelievers, although they did not think that it was
permissible.

End quote from Majmoo‘ al-Fataawa (7/273) 

The one who falls into such error must renew his faith by
uttering the Shahaadatayn (twin declaration of faith) and repent sincerely
from what he has done. As for the Shahaadatayn, uttering it in the prayer or
in the rituals of ‘Umrah is sufficient. 

It says in Kashshaaf al-Qinaa‘ (6/181): 

If the disbeliever prays or gives the adhaan (call to
prayer), he is deemed to be a Muslim, regardless of whether he was
originally a disbeliever or had apostatised. End quote. 

With regard to repentance, it is essential to repent
sincerely and specifically from that great evil. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
Whoever speaks a word of kufr, even in jest, has disbelieved. He must repent
and must believe that he is repenting from apostasy. So he must renew his
Islam, for the revelations of Allah, may He be glorified and exalted, and
His Messenger, are too great to be mocked or ridiculed.

End quote from Liqaa’aat al-Baab al-Maftooh (60/12) 

What you have mentioned about your friends is no excuse for
you not to alert them to the serious sin that they have committed; rather
what you must do is tell them about the ruling on what they have done, so
that they may repent to Allah, may He be exalted, from it, and they may
regret it and be careful not to go back to it again. 

You can read this message to them, as an answer to your
question, and similar answers having to do with the Islamic ruling on that. 

Please see the answer to question no.
153656
and
163627

And Allah knows best.