It is not possible to manage the subscription. Adding users is not possible.
The subscription allows a contingent of users. This can be seen in the team (e.g. 1/10). It is possible that the limit has already been reached. (e.g. 10/10).
If there are already users registered in the system with the plan "FREE", they can easily add the users in the team with add, if they fill in the email address field and ignore all other fields.
For brand new users who have not registered, it is important that they follow the conventions of the password (special characters upper/lower case). Fill in all fields.
In both cases you should refresh the browser (F5 refresh) after save/add.