This field allows to see the groups a user is in, and to filter items on a group. The field is only used as output field. There is no way to input a group from the field (because it's read from the system)
Requirement: The tracker needs a field user selector with option creator(1)
Ex: A tracker with 3 fields
7: name - textfield
8: user - user selector option 1
9: groups - user groups
If you want to list all the users of the group 'Members', you can use