Well reguardless of being on a team or by yoursef you are gaining XP on your own.
Being in a team makes the game a hell of a lot more fun and interesting. I think everyone should be in a team at some point to understand clearly... A well organised and respectable team if something you have to look for so anyone looking to join should do some research first.
Mostly everyone gets more XP on there own rather when they are teaming up. I do too. I've conjured together a great group of guys to form the WC and it was the best decision I have ever made in the world of GE. Playing is great fun now.