Passing user info from one site to another


I am having a bit of problem figuring out how to do this. I have two websites A & B, A is on Wix, and B is on somewhere else. B has the user login details and a link to A, and what I am trying to do is to use the same login details, or at least for A to pick up the email/user id when users go from B to A without them registering again. Does anyone know what’s the best way to go about this? Thanks!