Good question, thanks for asking.
The short list of mandatory items would be:
- Basic tools → whatever tool kit came with your bike, or whatever you done to replace that with “real” tools. IE, the usual hexes, sockets, drivers, etc.
- Whatever you need to fix your own flat, with the help of others. Tube, levers, patch kit, plug kit, air compressor, CO2, etc. IE, if you’re running tubed or tubeless, bring whatever you need to fix a flat.
- Water, food, snacks, layers, etc
The list of other items to supplement this basic set is endless. But the general rule is: no one expects you to know how to fix everything. Part of the the “adventure” of ADV is fixing / McGyver’ing stuff on the trail. And we’ve got some smart, handy folks in the club.
What’s not, cool, however, is to not have the basic stuff. For example, if you flat and went out on the ride without your own stuff to fix it, we’ll still fix it for you, but you’ll pay in shit talking and heckling 
And people who show up repeatedly unprepared with definitely get a reputation, and not a good one.
The additional short list of “good to have” items is:
- Jumper battery
- Quality tow strap
- Z-Drag system. I carry both of these, highly recommend Green Chili strap and Z-drag
Comms:
Finally, we’re big on radios here, rather unique in the ecosystem of CO riding clubs. Please review our Radio SOP here, and consider using this to get set up.
No Go:
Hard bags / luggage. Not allowed on our rides