In Spaces 5.1 controlled groups can be og_selective = OG_CLOSED

~line 965 within the definition of the controlled groupmask:

'og_selective' => variable_get('spaces_controlled_selective', OG_CLOSED),

This prevents spaces_og_subscription_link() from showing a join button, and also would prevent a user from being able to request access to the group (should they guess the url). og_subscribe returns drupal_access_denied when the group is OG_CLOSED.

Suggested change:
'og_selective' => variable_get('spaces_controlled_selective', OG_MODERATED),

or even this, because the variable spaces_controlled_selective is never set anywhere.
'og_selective' => OG_MODERATED,