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.