Account activation logic problem

[Yumi]

Again, haven't checked, but I'd imagine:

$mybb->user['usergroup'] = 5; // send cached user to the new group

// update $mybb->usergroup
global $groupscache, $cache;
if(!is_array($groupscache)) $groupscache = $cache->read('usergroups');
$mybb->usergroup = &$groupscache[$mybb->user['usergroup']];

Hope that helps.