I tried to join, and got this message:
"Sorry, you are unable to join this group. You are already an administrator of three or more groups. Please leave any other group, or upgrade to Premium Membership first."
I wasn't trying to become an admin. I just tried to become a regular member, and found that one can't do that. What this probably means is that you left the setting for regular members at it's default setting, which on DeviantArt is "applications not allowed" or something like that.
If you want more members, then you should go into the admin area, click on "manage members" and then edit the setting for "members"