Adding a User to a Group
To add an existing user to a group, do one of the following:

In the user tree, double-click a user, drag the group to the Groups section of the Details pane, and click Apply or press Ctrl+S.

In the user tree, double-click a group, drag the user to the Members section of the Details pane, and click Apply or press Ctrl+S.
Use Ctrl+click (Windows) or Command+click (Macintosh) to select multiple users, or Shift+click to select contiguous users.