Early on I made a couple posts in this thread to try to get the original poster to rid himself of the "we were here first" syndrome.

Much more has been said. Able brought up the;

AMA Membership Manual: emphasis added.
Important:
Once a Frequency Management Agreement is in place it does not
have to be renewed on a yearly basis. It is valid until one or both
parties (if AMA chartered club) disband, relocate, or sell the
property. The agreement is only valid for the specific parties named,
which means if a club disbands and another AMA chartered
club/individual member uses the flying site, a new Frequency
Management Agreement will have to be created, signed by all parties
involved, and copies sent to AMA HQ.
Participants in the Frequency Management Agreement are
responsible for informing any guest flying at the site about the
agreement and enforcing that only frequencies assigned for that
particular site are used!

First, IMO, the frequency-management tool is a good one for responsible people in flying-restricted areas. Like most tools, it cannot do EVERY job to the utmost.
Now I still refer to those non-club groups that just go out somewhere and fly without regard for others as "OUTLAW fliers." Simply my term for using simple language for description.

Outlaw groups come and go. They have as clubs do individuals that come and go. A frequency agreement cannot be valid for such a group as said group does not have recognized officers (as an AMA Chartered Club does) therefore as BM has said and/or alluded to several times, a frequency management agreement with such a group is (now my description again) more worthless than used toliet paper, good when unused, but quickly becomes just so much poop!

AMA provides for such situations that may arise. There have been, and I'm sure still do, model airplane clubs that are NOT AMA Chartered, yet are well disciplined clubs and will work to keep the RC portion safe.

First, meet with the group's main persons. Find out their needs. Set up a fly-in with the group as guests at your field. Serve up some burgers and trimmings. This has worked before and can again.

If you're in the Chicago-Land area, don't fly a model on park-land unless you are at the established flying sites. Same for Harris County here in TX. The rangers are nice but they perform as directed.

In that respect, if all common attempts to get together fail, then I do suggest you have an audience with the County Parks Department, not to blame, not to fault, but simply to present the advantages of having designated flying site/s, where all fliers meet and fly. Of course present and explain the advantages of the AMA insurance (especially being always in force for current members) and IMO, things will get worked out. Do not expect overnight results.

I have a number of good transmitters, only one being 2.4, two synthesized. I have some 25 receivers, only two being 2.4 and 2 synthesized. I will not be forced into giving up on my 72 stuff. It works great. I would never expect such of anyone.

2.4 does not and will not protect the park patrons. When Super Jock, goes to the LHS, buys that RTF and goes into the park at some open spot and launches the lethal weapon which immediately joins forces with the face of some bike-riding youth, then ALL RC will be banned. If in a designated area, RC may survive. RC has survived the chopper death in a designated facility here in Harris County some 3 years ago.

Edited; add emphasis