Only administrators are allowed to manage the network settings.
To modify your subscription :
From your profile, go to Settings
Under Network Settings, click Billing
Click Update Subscription
Select the desired number of users (in this example we go from 50 to 60 users)
A prorated invoice is automatically calculated*.
Click on Next
After reading, check the box "I have read and accept the subscription contract".
You will receive by email the invoice for the added licenses.
*The amount is calculated as follows: number of users added X price X number of months remaining until the next yearly renewal (to the day).