Hershel,
When a member of the Hunt Group is removed it will see how many calls are waiting in it. So if they are a non-member you would make them a member and then use the Remove/Replace or the Logout feature codes so they are not actually going to receive calls.
If they are a member of the hunt group and just want to pull the call from another members phone then using the Reverse Transfer feature and the Hunt Group pilot number of the Hunt Group they share may do it. The problem with this is that you may pull a call from another member of that Hunt Group that hadn't had time to answer the call, but was available without knowing it.
Unfortunately, if you pull the call directly from the members phone it may or may not be a Hunt Group call, but you would not know until you had pulled the call which is why I say to pull it from the Hunt Group instead. Also, don't forget to set the Group Call Pickup to yes within the Hunt Groups that you want to allow this on.
To answer you last question you cannot do a reverse transfer from an Agent ID, only extension numbers within the system; i.e. phones, hunt groups, phantoms, call routing announcements, etc..
Thanks,
TE