The above is for your next roadblock (assuming you have paid plans and don't want the invited users to pay also)
For having team plans, you build the app with the team billing flag
spark new project-name --team-billing
but I'm sure there is some way to turn it on after the fact.
Then in your Spark Service Provider booted method make sure not to have any user plans (Spark::plan) defined and have at least one team plan defined (Spark::teamPlan)