Putting out a call to upvote if you have a similar need! And any comment from Workfront on this is welcome. Custom forms allow you to add a typeahead field with 'User' as the object type. However, this only allows selection of a single user in a custom form. There's a big need to have a field that allows selection of multiple users (just like you can do in other places in Workfront 'regular' forms.)
After just discovering the typeahead functionality and realizing this limitation today, I am certainly going to upvote the idea! It seems unproductive to only allow one user to be referenced in this way.