First create a lookup for the list of contacts that you wanna add to the group. Then, click on the groups from the left navigation pane and go to the desired group.
Click on Add/Remove Contacts and click on contacts button from the window that appears. On the "select from" drop down, choose current lookup.
All the contacts that you have sorted using the lookup will be listed. Press on ">>" button to add all these contacts to the group. Click on OK once done.
This will add only those looked up contacts to the group. Alternatively, you can also use dynamic members as suggested in the previous answer as well.
Using dynamic member option will add future contacts automatically, based on the chosen criteria.
