You cannot log out of a ring group without logging out of the entire extension, but you can log out of a queue without logging out of the extension. This is done a couple of ways, but the easiest is by dialing *45 from the extension in question.
As far as ring strategies go, you probably want to use either linear or round robin. You could then assign penalties to the dynamic agents so that they always ring in a particular order.
See the documentation here for info on Ring Strategies:
I don’t use penalties in my systems at the moment, but from my understanding, if you set an agent to have a penalty of 0, those agents will be rung first in order of which they logged into the queue, the it will ring agents with a penalty of 1 in the order in which all of the penalty 1 members logged into the queue and so on and so forth.