I'm guessing you figured this out since this was from so long ago, but in case anyone else runs into this thread I'll throw this out there:
I haven't done this with a list of multiple users, but I have worked around this issue when creating a summary field that returns only one user - here is how I've done it:1. On the child table, create a formula field that converts the user field you care about to text, e.g. ToText([User])2. Create the combined text summary field on that new text field3. On the parent table, create a formula field that converts the summary field from a text list to text, then from text to user, e.g. ToUser(ToText([User Summary Field]))This method may be able to be tweaked to use for a user list, but I'm not sure exactly how it would differ in that case.