I just joined a few minutes ago directly from this page:
http://biology.polytechnique.fr/proteinsathome/team_display.php?teamid=1278
There's no "button" per se just a link that says "Join".
Cool - when I did a search for the team, the join link wasn't there, but your link worked... Go figure - I'm in now, so as soon as phase 3 kicks in, we'll be moving there, too!