You tried to invite a user to their Workspace in your Portal and see the "This account has no users left. Please contact your account administrator to invite more." notification.
There is a 15-day setup period on new Accounts where additional safetys are in place, Contact your Site Admin via their Workspace in your Account to have the 'setup period temporary user limit' lifted.
The safety for that setup period is typically 50 Users total (paid and non-billed seats).
Outside of the setup period, your Portal has no limit of non-billed users.
Should you need more than 15 Moderators on your Account, your sales rep will consult with you about whether to upgrade your Account to an Enterprise Account.