I'm very sorry for any inconvenience this may have caused. With the migration to our New Experience there was a change in how many users are available for an account. Customers who subscribe to the Email Basic plan are provided with one user, whereas, the Email Plus plan provides the option for up to three users. Any account with existing additional users were grandfathered in at no extra charge. Because of this you can deactivate an additional user, but would need to upgrade to Email Plus to add an additional user. So in your case, you can have a total of 2 users in your account.
Please know that account owners were notified several weeks in advance of the migration and changes (both within your account and via email communication). We'd also like to share that all customer feedback is being forwarded directly to our developers regarding this change.