If there is somebody who you want to join the system and be joined to multiple groups when they register then you can do so by going to the user index and choosing the Invite users by email address option.
You will be shown a form that lets you enter up to fifty email addresses of users that you wish to invite, choose the groups that they will be joined to when they register and enter a personal message to be sent to them inviting them to join.
If there are a large number of groups in the system then you can search for a particular group by entering part of its name in the search field. If some groups have been set as default groups that everyone is joined to when they register then these will be shown as already checked but will not be able to be deselected.
For information about allowing invitations, see How do I allow users to invite other users to join?