Hello @John_Winder,
You can have as many users as you want in your account. Each user can (based on your Make Plan and RBAC) access each scenario to collaborate. The only thing you cannot do is working on the same scenario simultaneously.
Or if this is a feature request: https://www.make.com/en/platform-ideas/p/scenario-builder-multi-user-collaboration-on-scenario-creationediting-or-session
Cheers,
Henk