Presently, SE offers the ability for Group members and Admins to invite general site members to Groups. When the invite is sent, it allows the invitee to see everything in the group they were invited too. If the invitee, does not react to the invite, this access status to the group remains indefinitely without recourse by the admins, i.e. the invite can never be rescinded.. This can be come a serious privacy issue for private groups. SE >Must< provide an admin feature to manually rescind invites and, or provide an adjustable TTL on group invites.

The problem with this scenario, is that social engine has no way of managing already sent invites even on the member level, let alone the group level. Only members can send email invites outside of the network. Not even admins can send invites to groups outside of their friends. Invitations are just a notification at present stating that a group is there, and no code really is in the link sent by email to members telling them of this fact or monitoring their progress.

What you really want, is an affiliate or referral system.

My site is brand new, so I have no worries about previous invites.

Also, on mainstream Social media platforms such as Facebook. The invitee cannot see the content of a private group 'till they accept the invite. This would also work to solve this issue on SE.