WakiPaki,
something's wierd with my computer and I can't access my Joomla sites right now to check, but I just looked at the mod_mygroups i put in the 1.5alpha2 package, and it seems I uploaded an old version. On line 38 in mod_mygroups.php, change #__gj_grcaterory to #__gj_grcategory, and see if that solves it. Can't do it myself right now, I'm afraid.