Sorry Garoth, but I would have to disagree with the part about how this ruins the game for people. I have had multiple new players join my team and mess up. Our team usually adapts to this problem, tells the new person what they have done wrong, and everything goes great. These mistakes are also good training. Experienced player learn how to deal with a problem such as a new person nadeing the base. It can also add some excitement to a somewhat boring game. If the humans and aliens are pretty much at a stand still where neither one can get to the other's base, then having someone taking out a few defences and letting a few aliens in, can help the humans stay on their toes.
It is a very rare situation when newbies doing stupid things are responded to gently and players actually help new players. From my experience there are four kinds of servers:
Newbies are absolutely untolerated/unwelcome (few)
Newbies are for the most part are ignored by the more experienced players who are too busy racking up kills(most servers)
Newbies actually get help on a regular basis (few)
And then there are the servers so laggy you can't tell the difference or everyone is messing around.
In smaller games I generally answer reasonable questions from new players and/or give them pointers to help them along. It's surprising how much explaining the dretch headshot helps.