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.
Regards,
Ronald
[EDIT]
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.