If you create a group you're asked how Membership request should be handled. 4 options are shown:
open - membership requests are accepted immediately.
moderated - membership requests must be approved.
invite only - membership must be created by an administrator.
closed - membership is exclusively managed by an administrator.
The 3rd option: Invite only, means an administrator should be able to either invite users to join the group, or (less friendly) just simply add users to a group.
The way it's now working: as adminstrator you can't add nor invite a user to a group in a simple way.
[Work around (already working, but not convenient at all):]
The only option is to enter this link in the adress bar: http://www.foobar.com/drupal/og/approve/[group_id]/[user_id]
Adding/inviting users as group administrator should be easy and convenient.
An add/invite-user-to-group-form, where you can select user(s) and group(s) to join/invite, accessible by link from the Group Details Block. This would do the trick.
The other (not working) way is to invite a user with this url: http://www.foobar.com/drupal/og/invite/[group_id]
Here you'll see a form where you can send users an invite over email by username or emailadres. The mail gets send, but when the invited user uses the invitation link, he'll gets to see an 'You're not authorised'-page. This is becouse the group is set as an "invite only"-group??? <- looks like some wrong access settings..