When adding $_POST['additional'] to my template at MemberGroups.template.php I noticed, that there is a broken line in SubsMembers.php.
In line 551 it shouldn't say:
CONCAT(additionalGroups, '$group')))
It should be instead:
CONCAT(additionalGroups, '$group'))
Am not sure if thats also in the 2.0 Beta 1, but I got this problem since 1.1 release.
I posted a
bugreport at the official simplemachines forums.