Yep, I got this problem too, but not exactly the same. I click the invitation and log in my developer account, and I even received a email says "Welcome to the apple developer program", but I just CAN NOT join the team. And I've tried for thousands of times, but it just don't work. But on the second day after I click the invitation I finally join the team.

OMG, I literally have the same problem. And I have called、emailed apple developer support team for several times for two weeks, but only one result: "Due to one or multiple reason balablabalabala......" We Seriously Need To Let Apple See This Problem mate!!!!!!!