Technically you need a license to transmit on any channel in the US. An Amateur Technician or General class is easy. Along the way to attaining you figure out where various resources are and how to search for them (like "2M band plan" will tell you that 146.520 is the simplex calling frequency, which ones are reserved for repeater inputs and outputs, etc...). A bit of teh Google can also show you what the GMRS frequencies are. However, it's only legal with the Baofengs to listen on GMRS because they are not compliant with constraints for transmission as set forth in FCC part 95 (though they will transmit on those frequencies). Down to purely practical matters a signal transmitted on any frequency not in use by a higher-power transmitter will be heard, other factors permitting. What you'd work out with your friends is called a "radio plan" and is as simple or complicated as you like. Ex: On odd days at 1300 I will listen on channel ___ for 10 minutes. On even days at 1800 I will listen on channel ___ for 10 minutes. or whatever. Something with agreed-upon frequencies that everyone knows about so that if you get separated there's a known time and channel to link up.