If I understand correctly, this is how it works out of the box. The user is asked to create a team immediately after registration.
All you need to do is stop users from creating additional teams is to add this to the booted method of your SparkServiceProvider:
Spark::noAdditionalTeams();
Be aware that to register your users on the same team, they must be invited by the owner of the team. If a new user registers they can create an entirely different team with the same name (personally I think this is not great, but whatever).