Is it permissible for fewer than seven people to share in a cow for udhiyah?

Praise
be to Allah

It is
permissible for seven people to share in a cow for udhiyah. This has been
explained previously in the answer to question no. 45757.

As it is
permissible for seven people to share in one cow, it is more appropriate
that it be permissible for fewer than seven to share in it, and thus they
will be rewarded for the surplus, and anything more than one seventh is
regarded as something supererogatory, such as if a single person sacrifices
a cow as an udhiyah, even though a sheep would suffice in his case.

Imam ash-Shaafa‘i
(may Allah have mercy on him) said in al-Umm (2/244):

If they are
fewer than seven, it is acceptable on their part, and any surplus is
regarded as a supererogatory act on their part, just as a camel is
acceptable on behalf of one from whom a sheep is required, and he is
regarded as doing a supererogatory act with regard to the difference between
the sheep and the camel.  End quote.

Al-Kaasaani said
in Badaa’i‘ as-Sanaa’i‘ (5/71):

There is no
doubt that it is permissible for a camel or cow to be sacrificed on behalf
of fewer than seven, such as if two, three, four, five or six people share
in the sacrifice of a camel or a cow, because if it is acceptable to offer
one seventh, then it is more appropriate that it be acceptable to offer a
share greater than one seventh, whether the shares be equal or different,
such as if one of them has half, another has one third, and the last one has
one sixth, so long as no share is smaller than one seventh.  End quote.