In SMA-SM Service Portal, if user wants to do "Request on Behalf", Tenant Admin needs to add the users in Source Group and Target group. After adding the users in the groups, it allows to use the Request on Behalf functionality.
Also, we don't see an option to mass load of the users to the groups(Source and Target groups). Many Organization contain more than 50k users and it's difficult to add users manually. So, it's better to provide the functionality to get the users by default in Request of Behalf option(Instead of adding users to groups) or provide the option to load the users in bulk(mass load).