Hi there! Hope you don't mind a stranger like me giving you some critique.

First of all, I have to say that I absolutely LOVE the composition. It's nice, tight, but not so much that the piece suffocates the viewer. Very nicely done.

Besides some minor anatomy issues, the main things you can work on are values and lighting. I can't really tell where your light source is coming from; the top? The side? And if it's from the side, which side? Top left? Top right? You can keep the lighting consistent by adding an extra layer where you draw arrows that come from the direction of your light source. Then you'll remember to shade accordingly. Also, you could try mapping out the values before you start coloring. This will keep your mind clear when you go about coloring.
Another reason why I say that you could work on values/lighting is the lack of shadows. What I mean by this is that the characters don't seem to cast shadows on each other. For example, if your light source is coming from the top right, then the guy with the green hair would be casting a shoulder on the girl, who would then be casting a shadow on the guy with the orange hair. If you remember to do this, the character interaction of this piece will significantly improve, and the people will look closer together! ^^ If you're thinking, "but I did add shadows!", then try making the shadows darker. If I can't tell that you added shadows, then they aren't distinct enough. Don't be afraid to go dark!

I hope this helped. Just focus on your lighting. Everything else is terrific! :]