At the top of the list, for me, is a Sat phone (I prefer the Iridium Extreme) and tourniquet (C-A-T or similar). The first item will keep you in contact with any EMS or in dire cases, AIr-Medivac. It deploys rapidly and will give you Lat/Long on the screen with a button selection to pinpoint your location. The phone also has an emergency declare button that you pre-program the service provider to notify two contacts of your choice. Show your group how to use it both and where they are located. If you go down, nobody will know where grab these itmes from your overpacked, "sherpa'd-out" stack of travel gear. The second (tourniquet) is self explanatory. Both items can be used solo. Access proximity inside the vehicle (this includes your first-aid kits) is super crucial and both units should be kept secured to a fixed part of the interior within the wingspan reach of your seated location. Ask anyone who has survived a vehicle roll-over. Head-rest monted first aid kits help but those too, can become separated from their secured position.
I travel solo frequently and these top two are golden while out on remote highways where they would be priceless to help assist, saving valuable resposne time(s) and lives.
PPE's
Then pack your kit accordingly to your available space/needs/isolation, remembering the ABC's and commonly used meds.
A - airway (OPA's and NPA's)
B - breathing (basicCPR) and the use of BVM's - bag valved masks, for the more the advanced/certified persons
C - circulation (blood loss management) and if someone in your group is predisposed to or has a known heart condition then perhaps an AED - automated external defibrillator
We spend alot of money on accessories for our rigs, why not do the same for when it comes to First-Aid.
For the adventurers without any training...
Whether you take a class on-line or locally, basic First-Aid/CPR/AED skills are invaluable for this type of recreational adventuring. This should be a prerequisite for any members joining a group trail-run, because if you are not prepared to render assistance to members in your travel group large or small, you may very well become a liabilty to their well-being in a critical incident.
Mental crisis-rehearsal for unplanned critical events is a big asset and will aid you in your response/preparedness.