OK, the elder makes you a member of the BOS (without a ceremony) but it's hollow. You can only wear the old armor he gives you or everyone will kill you, no one seems to recognize you as a BOS member in the BOS bunker, they still call you outsider and wonder why you don't have a guard. On the outside no one knows you're a member and the armor when you put it on says you're posing as BOS (I'm supposed to be one for heavens sake). The only perk you get is that you can purchase the better stuff. This svcks.
There should have been a way to make it more realistic and let the other BOS recognize you once you are a member, you should be able to wear the good armor without getting your head blown off, and the wasteland should be able to accept you as you but also accept you as a member of the BOS.
So far, joining is only worth the purchases, you can cheat to wear power armor.