If you were invited, then go for it.
Several years ago, before I met my husband, his ex wife, who was then his current wife, was attracted to a woman and the other woman was attracted to both of them. The had a few threesomes, then my husband backed out because he felt that the relationship should only be between his wife and this new woman. A very honorable this to do, but not the best thing, as it turned out.
In that situation the relationship could have been a triad type of relationship. As it turned out, the other woman felt rejected by my husband. This caused a strain between my husband and his wife as well as between him and the other woman.
My husband's fear was that the relationship would eventually be only between him and this new woman. And he knew how attracted his wife was to this new woman, and that she actually loved her. It can go both ways, but if it is just a sexual thing you are joining, then I don't see where there would be any harm in joining and having a good time.