Realize that if they CATCH you with any talkgroup programmed into their radios that you aren't authorized to have in there... it's bye-bye for you. They install what they want you to have, no more. There are plenty of groups in police systems that the beat units are not 'allowed' to have in their radios, for example.
You are intended to be listening to specific talkgroups while working, not 'wandering around' elsewhere. There will be a system record that you were on the other group, and yes, you have to transmit to get on it, with a real, legit on-the-system radio. When you rotate that channel selector, your radio transmits, says 'I'm on this talk group now', and the central controller acknowledges and affiliates you with that group. (among others). There is a log of every channel you affiliate with, and when.