Yes, requesters or users can be added to a project in Freshservice. However, the user must first be added as a requester in Freshservice. 

For both existing and new requesters, the Project Management option must be enabled in the Edit Requester page of the requester's profile to add them to a project. 

Note: A project license is required for requesters who need access to project management.

Enabling Project Management for a Requester