The word "gruscheln" was born from the combination of the German verbs "grüßen" and kuscheln ("greet" and "cuddle"). It was created for a company, the social networking site StudiVZ, which needed a term for the "poke" functionality on their site - the proper German translation "anstupsen" was already used by Facebook. I would therefore count "gruscheln" as an advertising neologism, and not as a "proper" word.
As a sort-of update to Tem42's writeup, I haven't seen or heard a reference to this word in many years. I would not recommend anyone to use it, you will probably get awkward laughter as a reaction at best. In the German language sphere, the word was never seen as cool or hip either. It was closely associated with the social networking site StudiVZ, and pretty much died with it.
I remember a comedian who made a joke about the word when it was still sort of new. He suggested that "greet" and "cuddle" was not what male students actually tried to do when they'd "gruschel" female users on StudiVZ. A more accurate word, he suggested, would be "grumsen" or "gricken", implying a combination of "greet" and "bang", or "greet" and "fuck", respectively.