On this tab, you can send invitation to the group. There are two ways of invitation:
•Invite existing site member - after selecting an existing site user in the User name field, an e-mail will be sent to the user's e-mail address based on the Groups - member invitation e-mail template; text entered to the Comment field will be included in the e-mail; the user can join the group either by clicking a link in the e-mail, or via the My sent invitations web part
•Invite via e-mail - this way, you can send the invitation to any e-mail address that you enter into the E-mail field; in this case, user will be required to register to the site after clicking the join link in the e-mail; text entered to the Comment field will be included in the e-mail
More resources can be found at Developer's Guide -> Modules -> Groups -> Overview.