When I sponsor a brand new user to the Fedora packagers group, their username does not exist in Pagure. I cannot add them as a co-maintainer on my package

Steps to reproduce:
1. A new developer registers an account in FAS.
2. I sponsor the new developer into the "packagers" group.
3. I try to add the user as a co-maintainer on my package.

Expected results:
I can type the user's name into the form and add the user immediately.

Actual results:
I do not see the user's name in the "add user" form (eg. https://src.fedoraproject.org/rpms/python-execnet/adduser). I initially thought I would have to wait a few hours for the systems to "sync up" so I could add this user. This leads to delays, loss of new contributors' enthusiasm, and feeling like the systems are broken.

Additional information:
After discussing this in #fedora-admin, mizdebsk and nirik pointed out to me that the user must log into Pagure one time in order to make their username present in the system.

I understand that it would be too much at this point to make Pagure automatically sync all FAS changes immediately.

I have an easier idea: Would you please update the text in the "add user" UI to explain that if you do not see a newly-sponsored user account in the drop-down, please have that user log into Pagure once?

If you point me at the code I can try to write something.