To add members to a group, go to the
Members section. To choose the members, press Select and a checkbox for each member will appear. Choose the members you want to add to the group by ticking the checkbox next to their name. You can use the All groups, All statuses and Search filters to help narrow-down the list of members that are shown.
Once you've got the right members selected, press the
Options button and then Manage groups. Any groups the members are already in will be shown with a dash in the checkbox, but you can ignore these. Groups they are not part of are shown with an empty checkbox. Click the group you want to add them to. The members are now added to this group.
Note: this will add these members to this group and leave them in the group(s) they were in before. If you want to move them, make sure you click their old group name until the checkbox is clear.