Membership - two levels a basic £x a year & an advanced level at £x per month.
Both memberships would belong to the same Groupjive group.
Of course, assuming GJ is present you can assign them to a specific GJ group based off subscription using CBSubs GJ integration.
Limiting signups - For example can cbsubs be used to book seats for a theatre/coach?
No, but CBSubs can be used to purchase tickets that would limit signups. CBSubs is a subscription management plugin, it's not a booking plugin or anything of the sort. You'd need a 3rd party extension that has "tickets" or something of the sort and handles the event signup. You'd use CBSubs to purchase tickets.