lots of newbies are asking how to grab, even i as confused at first, so heres a tutorial on How to Grab by CrazyMLC.
and before we start, press C to hold your character

1: Click on your hands (not your wrist), so they look like this.




and thats how you start the grabing process, heres a guide to acctually grabbing.

2: ungrab your hands (click on them), raise shoulders, contract pecs.
3: press space twice.
4: regrab your hands (click on them) and lower shoulders.
5: press space twice.

and thats a classic grab to the head folks, experiment yourself and try to find ways to beat your enemy through grabbing.