8
Are you aware that they offer a PLUG-IN for XOOPS on their site: (the news article car linked to mentions it in a slightly confusing manner)
http://membership.cgi-central.net/scripts/amember/features.php#cm01
I mention this as there is the amember script (one price) which you can pull your hair out trying to integrate and then, there is the XOOPS plug-in, which synchs XOOPS and amember users for a single log-in.
If you in fact have paid for both just tell the folks at cgi-central to fix it, if not I would suggest the plug-in.
Good luck and hope I'm not stating what you already knew.