What "user names" would these be? The names returned by Get ( UserName ) ? for each user and which are specified in the application preferences for each install of FileMaker?
There's no way to automatically extract the complete list of user names nor, for that matter, the complete list of Account names defined in manage | Security.
You can, however, set up and populate your own table of user names and then your value list can list values from this table. You can even set up a script that runs when the file opens that compares the current user's user name to the records in this table and creates a new one for them if they are not already logged in the table.
This is not the best answer although it might be the correct one. Filemaker needs to start exposing some metadata about the system at runtime for the purpose of developers using that in such a way as to make the system more dynamic. Other systems and application development environments call this introspection. I understand the limitation, but it would be good to have a function that returns a list. For example, there should be a way to dynamically return data from a REST call to a dynamically created temporary table space.
Well that's what product ideas are for here in the forum community. If you would like to see this added to FileMaker, switch over to that section and either vote for it, if someone has already suggested it, or post your own idea if you can't find one you want to vote for.