Canyoneering is a remote sport where medical help is often not readily available. If someone in your group gets injured and you're lucky enough to be able to summon medical help, most medical help takes at least 4 hours to arrive, often longer. In many of the accident reports I've read, the injured party is forced to spend the night in the canyon before search and rescue can extract them. Therefore it is critical that every member of your team is familiar with at least the basic first aid skills.
I am not qualified to teach first aid skills, so this canyoneering guide will not go in depth on first aid skills. However, I recommend that at least two members of your group have an in-depth knowledge of first aid and everyone else knows the basics.
Each member of the team should carry a personal first aid kit, and at least one member should carry a group first aid kit.