If you're a Team Owner and need to update your subscription to invite more members, all upgrades are available under "Billing" under your user profile in the top menu.
Under Profile >> Billing you can now just select the "Manage Subscription" button then using the drop-down menu select the TOTAL number of members you need. To confirm select "Update Subscription".
The system will take a minute or two to update, then you will be able to add the new person to your team. On the Group Manager page invite the extra Team Members to your team using the "+Invite Member" button.