Before a licensed client can be added to a task they must be added to the project as a resource. This can be done by selecting the resource tab via the project detail page. Once the client has been added to the project, anyone with permission to edit the project plan can assign them to a task.
It is important to note several things:
1. The client user must have access to the project application. You can provide users access to applications via their record in admin.
2. Client users who are assigned to tasks can only make a comment. They do not have the ability to update the tasks status.