Hello,
Do you use iceScrum Cloud or iceScrum Standalone (on your server)
I am not sure that I understand your issue, maybe this can help you:
There is no way to register user through team/project management but if the option is enabled on the server, you can invite them by email.
Otherwise, users needs to register themselves or to be created by the iceScrum administrator (feature only available in iceScrum Pro) beforehand, and then you can add them in your team / project.
Regarding roles, a user cannot be team member in a team and product owner on the project which is associated to the team. The only project / team role combination that is allowed is Product Owner (project) / ScrumMaster (team).
