In the Pro Plan, if your account is “linked” to a group this means that the billing is handled together. One payment covers the combined total Employees in all linked accounts. This allows each account to schedule separately, while taking advantage of group pricing.
When a Manager on an account goes to their SETTINGS> Create a New Linked Account page and creates a new new linked account for the first time that also creates a “Billing Group” subscription with a separate account number.
Then in the future any number of linked accounts can be added on that page by any linked account Manager.