Do you think everyone who uses the app will have to create a ‘new’ user ID under the Humanitarianresponse.info server?

No! As a project owner, I would recommend you to have 2 user accounts on the same server (whichever you wish to have based on your needs and requirements). Here one account should be the admin user account. You will be uploading and managing your survey project through this admin user account. It would be best if you never shared the login credentials of your admin user account with anyone else (security reasons). Then the other account would be the enumerator's user account. You will need to share your projects from the admin account to this enumerators account (restricting various access to your needs and requirements).

Basically, you will need only 2 user accounts, and it’s you who should create this. However, if the enumerator wishes to collect data through their own user account, you could also use their user account as an enumerator’s user account.