To add a distribution list into group(s), login to the MSPControl and click on Distribution Lists,
or go to Organization > Exchange > Distribution Lists.
You will see all Distribution Lists. If you have many, it might be a good idea to use Search form in the right to find the one you need. You can choose whether you want to search by Display Name or E-mail Address.
Once you found the list you were looking for, click on its display name.
You will see settings page with five tabs: General, E-Mail Addresses, Mail Flow Settings, Permissions, and Member Of. Go to Member Of tab.
Select group(s) in the list and click Add.
You can also delete this distribution list from groups. To do so, check the corresponding group(s) and click Delete.
Don't forget to click Save Changes!