What are you actually trying to do here? It seems to me that we're close to Parent Child or Major Incident type behaviour and as such you don't want to create a list of users, you actually do want to create child incidents, just doing it quickly.
What I need is a list of users and their email addresses in a seperate field which I need for a trigger.
I just want to give the analysts an "easy" solution how to choose/select the users...
typing in the email addresses into a seperate field will work already but isn't a nice solution...
cause my trigger sends emails to known and unknown users...
and for the system known users I thought a multi-select box will be nice ;-)
Did you ever find a way to get this working as I am trying to set up something similar myself?