Extending person picker to show only user in selected groups


We are trying to extend the person-picker form field to accept a new group parameter from designer,
we already did that part where admin can now type in a group name and we are able to capture the new field in the task app.
1- added a new field type called people of group
2- added a new param called group for this field
3- in task app we know the selected group is “tester”

Now we are stuck on querying only users from the group “tester”, we played a little bit with some functions but couldn’t figure it out,
we found that the function UserService.getFilteredUsers() is responsible for searching the users, and it has param called restrictWithGroup which can contain a groupID (Please correct me if it’s wrong) but didn’t understand how we can pass the group parameter to this function. even passing a static value as this parameter does nothing and the result is not filtered by group,

Any help would be appreciated.

also curious about this