Usually, Nuxeo Studio exposes widgets that deal indifferently with users or groups.
In the case of aggregates such as selectManyListboxUserAggregate, it seems this is not the case.
Unfortunately, some fields in custom documents do hold a reference to a group so that I can not use aggregate features on my customized search. What a pity !
It seems it is possible to twist the selectManyListboxUserAggregate with the following parameters :