Guys are more than willing to admit that they love the fact that you get along so well with his friends. Why wouldn't he be?
Knowing that you don't mind hanging out with him and his friends on a guys' night certainly makes him at ease because he doesn't have to report to you like he probably would when he gives you the lame "its a guys' night" excuse. This way, you're there to witness it and he doesn't have to do any explaining.
On top of it all, he likes knowing that his friends might even wish that they had such a wonderful girl to call his own. He likes being the envy of the group.
BUT be careful with this and learn how to draw the lines!
Sometimes, a guy really does need his time with just the guys. And eventually, the friends are going to wish that you weren't always there on your man's arms. Its not that they don't like you, but they need the space from you just as much as your guy does.
A hang out with the guys is fine every now and then, but remember... you've got a life of your own and girlfriends who would probably appreciate and enjoy your company even more!
(Besides... girls have more fun anyway!)