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.)
Here's the topic on the Idea Exchange with 220 votes so far - please upvote! https://one.workfront.com/s/idea/0870z000000XhXXAA0/detail
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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.