Provide a way for existing users to invite new users into the system

[mitar]

This can be useful to spread the use of the platform. This should be possible only for open groups (and when we allow registration).

When inviting, we should remember where was user invited and then redirect them there after accepting invitation.

Few places for invitations:

• to participate in a discussion
• as a menu entry in the user's menu
• to be a delegate