If you know the subject material - ie. you can generally go off topic, and field a few non-standard questions (you don't have to be a walking mrexcel.com) then you can train others.
The only problems are being comfortable talking in front of others and having the correct training structure.
If you know the people you are training - my guess is that you are in the same team - then they won't be judging you in the same way a stranger who had paid for a course would.
So you are left with the structure of the training and an earlier post alluded to a concern about throwing people in at the deep end.
I have trained a few people on starting with VBA, and I see it as 'sowing the seeds' - its then only people's desire to know more (by coming to mrexcel.com / books) and their imagination that really moves them on.
I started by showing them around the editor and a few key buttons. I then set up a spreadsheet with some football results (it was a team of guys) and showed them how to change the font colours and then write an if statement to determine home win / away win / draw using absolute references.
I then took it a stage further to demonstrate the same thing but with variables so that they could then have a loop a do all of the matches instead of copying the code.
I hope that this might help to show you where to start and am happy to send the training files I use if you PM me with your email address.