it could give you extra charges, not your friends.
I txt my friends on AIM too, but my Mom made sure that me texting on AIM, is the same as unlimited txting.
You should make sure that you have it on your plan first, and then you can be sure that you or your friends won't be getting charged. Talk to your parents about putting AIM as Unlimited txting on your plan, and you should be fine.
All 3 of them FAIL! There phone just reads it as a regular text message So if he can text 500 other text then he can receive 250 and send 250 or if he gets unlimited from any network then there nothing to worry about.
That all depends on their cell phone plan. If they have the unlimited texting than it doesnt matter where the text comes from, its free. But if their plan isnt unlimited and they have say 500 'outside' messages than they are only allowed that many outside of their network