Why are my users not appearing in the ‘assign’ field for tickets?

In order for new users to appear in the assign field, they must have logged into Trac at least once. It is the act of logging into Trac for the first time that updates the assign field (i.e. just creating the User by itself is not sufficient).

So generally the procedure for adding a new User account would be:

1. Create the User and assign the password in the Control Panel.
2. Log in as this user in Trac (or get them to do it!). Normally this would also be a good time to hit the Settings link in Trac to set up their email address if they also want to receive email ticket notifications.