Technically, all the pilots met each other in EP 6. They were all lured to New Edwards Base on their missions. The EP I have listed are when they actually know and talk with each other.
The gundam pilot of Wing. Quatre "met" Heero in EP 8. Quatre gave Heero instructions for the mission of destroying the missles. But, Heero actually "knew" Quatre in EP 24, when he was forced to fight Quatre in Wing Zero using Mercurius. Quatre often advises Heero not to fight and protect pacifism. Heero and Quatre both care about outer space in a similar way.
The gundam pilot of Deathscythe. He "met" Quatre in EP 10. Duo saved him from being crushed by a train during the mission to destroy the Tauruses in Siberia. Their friendship is closer than that of other pilots since they both have friendly natures.
The gundam pilot of Heavyarms. The first pilot Quatre met. In EP 2, they fought each other and Quatre realized that it was wrong. So he surrendered first then Trowa surrendered. They both play musical instruments and sound great as a duet. (Trowa plays the flute while Quatre plays the violin.) It seems that Trowa understands Quatre the best and often teaches and helps him whenever Quatre needs his help.
The gundam pilot of Shenlong and Altron. These two had only a few encounters. They finally "meet" in person in EP 43. But they really "met" in EP 17. When Wufei helps Quatre and Duo just in the nick of time because they were lacking fighting power. With Wufei's independent personality, Quatre has a hard time trying to be his friend. They did talk in Blind Target though. Wufei told Quatre that he was strong and should never have doubted that because he has Sandrock. (The gundams only acknowledge the strong in Wufei's opinion.)
The representative and princess of the Sanc Kingdom. They first meet when Quatre goes to the Sanc Kingdom with Heero in EP 31. Quatre looks out for her when Heero is out on missions. They both pursue peace and have similar goals but they use different methods. (Quatre calls her Miss Relena ^_^)
A guard of Relena. She's quite good friends with Quatre when they first met at the Sanc Kingdom in EP 31. Quatre soughts her for advice and guidance. (Quatre calls her Miss Noin ^_^)
They first met in EP 31 but they didn't talk to each other until EP 44. Seeks for her friendship but she refuses and choose to fight and queston his ideals. Quatre believes that she is kinder than him.
A friend of Trowa's. They first met in EP 38. She made Quatre depart so he would leave her and Trowa alone. She seemed to hate him in the beginning but began to understand better when Quatre saved Trowa in EP 43 which helped Trowa regain his memory.