HI Mehran,
If I understand your request correctly you want to view the users in a group from the user picker. If that is correct you can't do that natively and you would most likely need an add-on such as ScriptRunner to populate a custom field with that data.
In the meantime you can also submit a Feature Request on JAC for this behavior. If you do please post the link here so the Community can vote on the issue and add impact.
Cheers,
Branden
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