Well, I am NOT going for the short answer. And this reminds me of something said about ham radio operators.
I there were 2 ham operators in a city, that city would have 3 ham clubs so each of them belonged to a club that the other one didn't.
That being said. I fully agree with what has already been said. Problem is that these groups are the self appointed types that have little to no outside support and do little to support government entities. Reason I say that is simple, if they were working FOR some agency, the agency would provide them the ability to communicate. If they have setup shop on GMRS, then they are one step removed from ham radio and wanted to go their own way and not deal with hams either.
Which brings about the second possible cure that will never work due to mentality. And that is SHARED resources. One big repeater that multiple groups use for communications. And if you run a community repeater controller that is multi-PL enabled, then they don't even hear each other talking. We did this for YEARS in the LMR business. Put a few high profile repeaters up that were setup as community repeaters and sold air time on those repeaters. Not real common any more but it was effective. But we are back to the problem of getting people to SHARE. And that is where you are gonna get stuck.
The third way to begin to deal with it is system design with limits on coverage area in mind. Directional antenna systems, down tilt, decreased power (which ALWAYS pisses people off when you mention it) and getting the system owners and users to spend money for real radio people to design and install their systems. Problem here is again money. Ham radio is by far the cheapiest way to communicate for SAR and similar groups. You are using repeaters that are typically owned by others, the license are cheap and the radios are just as cheap. But unless it's YOUR ham repeater, you can't have exclusive use of it. GMRS does get you a bit closer, has no test to pass and can be a cheap. But again, if it's not YOUR repeater, no exclusive use. So they spend as little as possible to put up as much repeater as they can so they have exclusive use of it. And I know that's how they treat it because if they were sharing use with other groups, we wouldn't be having this discussion. Actual designed and implemented systems that are tailored to cover a specific area are expensive. Not so much the hardware as the design of it. That requires knowledge that a typical GMRS operator frankly doesn't posses. And that's not a dig on GMRS operators,,, it's just a fact. So by the time you pay for all the design and increased cost for the antenna system you could have dropped 500 bucks and gotten a LMR license and been done with it. Then you have exclusive use of your own repeater pair and can do what ever you want. But they are too cheap to pony up the 500 bucks for coordination and a license fee.
You are NOT gonna get more frequencies for GMRS. And license holders have equal access to the allocated frequencies. And contrary to popular belief, putting up a repeater with a different PL or DPL on the same frequency is NOT purposeful interference. By putting a different PL on it, you have shown effort to mitigate interference and you have just as much right to use that repeater pair as the other guy does. Now this of course requires that the other repeater is closed access. But at that point, you are left with no other choice. So letters to the FCC are pointless.