If you are a group administrator and go to the index of members for the group then you will usually see two different buttons used to add new members.
- The Add users button lets you browse through the index of users and select those that you wish to join. Note that this button will only be shown if you have permission to view the user index - some sites may restrict this for privacy reasons.
- The Invite button lets you invite people to join by specifying their email addresses. This button will always be shown to group administrators.
If you have permission to activate users then each user chosen will be immediately joined to the group. If you do not have this permission then they will instead be emailed an invitation asking them if they wish to join and a task will be added to their tasks which lets them accept or reject the invitation.
If you used the Invite button and gave an email address that does not already exist as a user then what happens next will depend on whether the system allows outside people to register or not.
If they are not yet a user and guest users have permission to register to use the system then they will be sent an e-mail with a link taking them to the user registration page. Once they complete the registration they will be activated as a member of the group without requiring a response to a further confirmation e-mail. If they have been invited to join more than one group before they confirm, then registering will join them to all those groups. If user approval is turned on in user workflow, then they will still need to have their user approved before it is activated.
For more information on allowing invitations see How do I allow users to invite other users to join?