Your radio can only have 1 smart group in the UR field at any one time, so you can't subscribe to multiple groups. The routing is to your callsign. Subscribing is a matter of putting the callsign of the smart group into the UR field and kerchunking. Wait to hear the notification that you're subscribed, wait a little to see if there is a conversation already in flight, and then key up to talk.
You leave the callsign of the smart group in the UR field so long as you want to continue to use the smart group. Only need to enter the disconnect callsign when you want to unsubscribe from that smart group.
No default necessary on a hotspot as it's all controlled from the radio. :)
