When groups are linked together you have the choice if to create individual subscriptions for each group or create a subscription on the Group Collection which will cover all groups in that collection.
Having a subscription on the Group Collection means you can create an unlimited amount of groups within that collection. This can be more economical if you need lots of groups around the same set of members.
You can start a subscription on your Group Collection directly from the billing page. If you already have single group subscriptions for some of your groups, when you create a Group Collection subscription the single group subscriptions will get cancelled.
If you have a Group Collection 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.