As the group admin, you're able to add seats to your current group at any given time.
To add members to your group subscription:
- Ensure you are logged in as the group admin.
- Navigate to https://their.substack.com/account (replace "their" with the name of the Substack you're subscribed to).
- Scroll down to "Manage group".
- Type in the email address of the person you'd like to add.
If you've filled all of your seats and are adding an additional member to your group, you will be charged a prorated amount. You will be greeted with a new window asking you to confirm the prorated charges: