As Sarah said...
Many pirates dislike the extra help when they are leveling. When you both hit the same enemy, that person misses out on the maximum rep available. By joining a crew you get blank % back of your total rep earned. For instance, if you get 100 rep and your have a 25% crew bonus, you get 125% rep. The guides Sarah listed will go into more detail about that.
You also earn more rep by using the skill that does the least damage. So if your using your voodoo doll you will get more rep by using "poke" the whole time than if you used "swarm". So if your using your grenades and taking away a good portion of the enemies life, that also takes away from maximum possible rep.
My suggestion is to find a crew or guild to rep with or to help. Many helpful pirates out there would love to assist you with leveling and/or questing. All you need to do is find a server where the group of enemies your looking for aren't being fought. Once you find a "quiet" server ask a friend if they wouldn't mind healing you so that mob doesn't tear you to shreds
. And if you come across someone who looks as if they are leveling just ask if it is alright if you tag along. Some pirates won't mind the company as long as you use minimal damage skills and join the crew to make up for the lost rep. Other pirates may not want company in which it is best to just find that "quiet' server. As time goes on you'll eventually catch on to the "unsaid" code of POTCO. If ye ever need a bit of advice feel free to send a PM my way
Hope this has helped some..