To add members to a distribution list, 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.
To add members, select them from the drop-down menu and click Add.
Please note: you can add both mailboxes and distribution lists.
When you're done here, click either Save Changes or Save Changes and Exit. The first option will leave you at this page, the second will redirect you to all Distribution Lists.