In Settings > Plan and billing details you will be able to see number of Users and Partners that are being charged to your account:
To change the numbers of subscriptions you would go to "Settings > Users" to change number of Users, or to "Settings > Partner Portal" to change number of subscribed Partners. Changes you make there will automatically update your Plan and billing details and will apply with the next invoice.
Changing number of Users
Go to Settings > Users. To add more users to current subscription plan go to "+ADD NEW" button in top right and enter users details. Email with login details will be sent and user will be added to a list.
To remove user(s) from your organisation mark checkboxes in front of their names and go to "Actions button" then select "Delete". Users will be removed and their access disabled.
Changing number of Partners
Same applies for Partners - they are modified in Settings > Partner Portal. To add new one use "+ADD NEW" button and to remove them select checkboxes in front of their names and hit "Actions > Delete".