To my knowledge, there are no existing plugins that allow you to manage service-subscriptions. You can, of course, create that yourself.
It’s hard to say what amount of power you need for a particular group of people. It very much depends on the amount of people that’s concurrently online, the amount and type of plugins that you have loaded, the clients that are used, the configuration of Openfire, and many other factors. That said, with an average server, you should be able to have many thousands of concurrently online users in Openfire.
You can go through that thread (a user has conducted simulated benchmarks, he also mentions hardware) OpenFire Single Server Load Testing 120,000 Concurrent Connections
As Guus told, there is no way to predict what hardware you will need based on users numbers. On one server users may be very active, have a lot of rooms, on another they may send just a few messages per day. Nobody here will be able to give you the exact number for specs. You will need a lot of RAM. CPU is less critical usually.
Again, we can’t tell how many resources will you need (even based on these tests as they are just a simulation). Yes, RAM will probably be the first thing you will need to add more. You also don’t need that much bandwidth, but this is probably comes as a part of standard package.