You may be thinking of this kind of opening: "Hi, Fellow Trumpet-Players, What do you think about the latest trumpet music composed by ....?" This would work (in the UK at least) if you were posting something on a trumpet-players' forum. In England, "hi, mate" and "all right, mate?" are quite common greetings among friends. If you don't know the other person, it could be OK but some people might consider it over-familiar and a little rude. some people address other men as "fella" (from "fellow") but it's quite informal and I wouldn't recommend it to a foreigner.

Neither, unless you speak Australian English. Then "Hi, mate" is correct. The word "fellow" is never used to talk directly to someone, but only used to talk about someone. Example: "Have you met my friend Alan? He's a fine fellow." But then again, this is a little antiquated - meaning you sound like a very old person. This is not a bad thing - in fact I think it's a more pleasant way of speaking than we hear normally today.

To build on Mr. A's response, here are some informal American slang greetings: Hey dude Hey man Hey bro What's up dude These greetings are usually only used among younger people and should not be used in a formal setting.