Most memberships you can join by pay some fees do NOT count. Of course, it doesn't hurt either. But for those cheap one, don't count on it.
For Sigma Xi, as far as you have 2 first authored peer-reviewed papers (not conference presentation), you are in. It is easy, thus doesn't count. I am a member , joined just for my NIW. But for Eb1a, it mean very little, if any.
Good luck.