Create a mirror (NP) entity of the User/Account entities.
Something like this:
You can allow an anonymous user to write to this entity.
When the user submits the signup, you can create an account in the microflow according to the entered values.
This works, since by default a microflow doesn’t apply entity access and thus the creation of and writing on the account/user attributes is possible.
NOTE: Be aware that this is the most minimalistic way of creating accounts by anonymous users.
I would recommend to use the forgot password module https://appstore.home.mendix.com/link/app/1296/
This requires an anonymous to confirm their account and gives more control over account creation then just creating an account as long you specify a valid username and password.
As an admin, you can create the account, and sent logins to the user.
System.user is indeed designed so it can’t be modified or altered. Using an ‘Account’-entity is a good idea, I guess you good make a link on the log-in page, with a text, something like: ‘create new account’, and refers to a new page where a user can create his/her own account.