Aside from all the other valid solutions/comments: Are you running the project on your own? Everyone in the project automatically becomes a user as well. So your Mendix account is already 1 extra account, compared to the 6 local ones you have. If you have more team members, their accounts are also linked to the app and regarded as a user.
The free version has a maximum user base of 10. If you need more you need a paid license. The user base is calculated both from the local users and the appcloud users.
To double check the named accounts download the database and check with postgres the number of users in the table. If you have less then 10 active named users I would file a support request. But my guess is that there are 10 active users in that table.
Also, see René van Hofwegen's comment in this thread how this limit is calculated.