Yes, you may. While it may not be a sacramental marriage, a wedding is a wonderful time to share your friends’ joy. As you would at any other celebration with other Christians or people of other religious traditions, respectfully attend and participate to the extent that you can in conscience.

Most other traditions don’t have communion as a part of their marriage rite, but if they do, politely decline since it would be dishonest to participate in communion in a church with whom we are not in full communion. By the way, I find that a table setting always makes a nice wedding gift. That way they will always have a place for you at their table when you come to visit!