Firstly, if you're the leader, it's probably a good idea to have a current First Aid Certificate. Without the training that goes with this, you'll have difficulty dealing with some potential problems.
When something goes wrong, as usual the first thing is to avoid panic. This website will not attempt to teach you first aid. However it is worth remembering the St John Ambulance response plan in an emergency: D-R-A-B-C. To apply this properly, you need to do the course, as you need to be able to provide EAR and CPR if required, and identify a range of other things that could be wrong.
D - Danger - to self, others and/or casualty
R - Response - to voice and touch
S - Send for help - Dial 000
A - Airway - is it clear and open
B - Breathing - is the casualty breathing
C - Circulation - Check pulse in neck
and a new one which isn't much use in the bush, D - Defribrillation
You can find D-R-S-A-B-C-D here.
The most likely events requiring action are probably blisters, tiredness, headaches and the like. Work out what you are going to do about these. The most likely more serious incident is a fall. Twisted ankles are quite common, as are bruises and scrapes. Eventually, if you spend enough time in the bush, you or someone with you will injure themselves enough to require assistance in getting back to civilisation. You can carry a small First Aid kit which will deal with many of the more common problems.