First make sure you purchased enough licenses on the Subscription page.
You will be able to change the user count at the bottom of your plan if you are currently subscribed to a Pro or Business plan.
We then display an Instant fee which will be charged right away according to the progress of your current subscription period.
Your Next payment will be automatically adjusted to your new user count.
If you add more users in your Members settings you will be automatically redirected to your subscription settings to add more users.