If one is keeping the 1st half of the omer then if your invited to a wedding before rosh chodesh, can one go or is he required to rather keep the second half?
It is permitted to go to the wedding, and you do not have to keep the second half of the Omer. According to some authorities one should not participate in the dancing and celebrations.
Sources: Iggros Moshe (Orach Chaim 1:159; 2:95; Even Ha’ezer 1:97) writes in a number of places that if the family making the wedding do so be-heiter (with halachic permission, for example if they keep the second half of the Omer), it is permitted for anyone to participate, even those who keep the first half of the Omer. Others are stringent (see Minchas Yitzchak 4:84), and some write that one may ‘drop in,’ but should avoid participating in the celebrations (Rav Wosner, Mibeis Halevi, Vol. 1, p. 56).