When groups are linked together in an Organization you have the choice if to create Single Groups subscriptions for each group or create a subscription on the Organization which will cover all groups in that Organization.
Having a subscription on the Organization means you can create an unlimited amount of groups within that Organization. This can be more economical if you need lots of groups around the same set of members.
You can start a subscription on your Organization directly from the billing page. If you already have Single Group subscriptions for some of your groups, when you create an Organization subscription the Single Group subscriptions will get cancelled.
If you have an Organization subscription and you add additional members over your current subscription member count, the subscription count will increase automatically and the difference will be taken into account at the next billing cycle. This is explained in more detail here.