On this tab, you can add a user to a group directly, without sending an invitation. The following details need to be entered:
•User - select an existing site user who you want to add to the group
•Comment - text comment that you can add to the user; this comment is not sent to the user, it is only displayed in the administration interface
•Approve - if checked, the user will be automatically approved; if not, users will need group admin's approval before they become members of the group
•Add roles - you can use this button to assign the user to group roles; a dialog where you can select from a list of available group roles is opened
More resources can be found at Developer's Guide -> Modules -> Groups -> Overview.