To add more members to your paid subscription you can go to:
Login to your account
Billing section under "Settings"
Manage (on the right)
Manage (in the Subscription area) then "Change Subscription"
Scroll to the bottom and click "Next"
Check off "10k Members" then increase to whatever quantity you want
Click "Next" then confirm your changes
You'll be taken back to your admin portal and now have more members!