There is no specific collective noun for brides, most likely
because brides seldom form groups. However, if you need a
collective noun for a rare grouping of brides, choose a noun
suitable for the context. I have seen suggestions for a bouquet of
brides (charming), a giggle of brides (borrowed from a gaggle of
geese), or a babble of brides (lots of arrangements to discuss when
you group prospective brides).