ANSWER:

If such gatherings are segregated and no one would see her except other women, there is no problem in that.

But if there is a possibility of a non mahram man to see her, this is totally prohibited.

QUESTION: In my place people book hall for marriages, and invite many people, the woman who is about to marry puts a lot of make up and wear best clothes is it permissible? Or should she be wearing abaya?