If you signed up for the service, you will see a screen like this:
If not, it will tell you who your administrator is, and you can go and hassle them!
If you are the admin, this is where you can invite new users, remove users and buy more licences.
Click "Edit Organisation"
You will see a screen like this, which tells you the current state of your subscription
If you need to add more licences, click "Manage Subscriptions"
If you want to add more users, click "Invite Users".
Please note at the moment, you cannot bulk invite users. You have to invite them one-by-one