You can set quotas for external or CommCell user groups. A backup user who is a member of a group that has a quota inherits the quota from that group. (Quota Calculation explains how a user's effective quota is calculated.)
Adding an Owner to a Client for File System Backup Quota
To enforce a quota on a client computer, the client computer must have at least one owner.
To assign a user as an owner of a client, use the domain_name\user_name format and follow the steps in Adding an Owner to a Client.
Note: To be considered for quota calculations, users in an external group who are added as a client owner must first log in to either the CommCell Console or the Web Console at least once.