There are several ways to link subscribers to a group. Here are all the options:
1. From the group
You can link subscribers to a group from the drop-down menu on the right of the group list. As with segments, you can add the conditions that you want subscribers to meet in order to add them to the group.
2. Using a segment
In the Segments section under Manage contacts>Segments
you can link subscribers to a group from the drop-down menu on the right hand side of the segment list. "View>Add subscribers to a group"
or from the segment detail, by clicking on "View", and on the next screen by clicking on the drop-down button "Edit>Add subscribers to a group"
Finally, you must select the group to which you want to assign the selected segment.
3. In the list of subscribers
From the subscriber list in Manage Contacts>Subscribers
You can filter the subscribers you need, choose them individually or by batch and then click on the "Actions" button and select "Link to a group"
4. Adding contacts
Whether by importing your contacts using Excel or adding them manually, you can assign them to one or more groups.