Permissions for assigning a case to a different user

[ps40]

We recently upgraded from 5.x to 9.x. We are using the Google integration extension to import emails from a gmail inbox and automatically create cases. Sometimes these cases need to be manually reassigned to other users in the system. Currently no users are displayed in the dropdown. This used to work in 5.x. I am guessing I need to add some sort of a permission but can't find out which. I have looked at the documentation as well as the forum but didn't find anything. Please help.