This would include all users, not just ones who have posted in the current topic. But the current topic's users would take precedence.
This would probably use https://www.mediawiki.org/wiki/API:Allusers . I think the only choices are "Alphabetical order starting with a given string" or auprefix ("Search for all users that begin with this value") (probably what we'll use).
I don't think we can do case-insensitive full text search the way we do for topic posters. Question: Should we then change topic posters to also be prefix search to be consistent?
There are various things we could consider, e.g. auwitheditsonly ("Only list users who have made edits").