Grom is a derogatory term used to refer to a newb, poser or wannabe. Typically this is someone new who doesn't know the ropes but also who is trying very hard, often too hard, to fit in. Their lack of true knowledge, regurgitation of incompletely understood themes, memes and ideas and general inexperience coupled with an incessant need to be perceived as cool cause annoyance to many other members of the community.
Mopeding, like many tight communities based on a common set of interests, involves its own set of merits. These include possession of fast attractive or interesting mopeds, in-depth knowledge of moped maintenance and performance tuning, being fun funny or otherwise enjoyable company, and having true love of moped riding and culture. A person may be labeled a grom for trying to fit in with persons who have earned moped respect by claiming, without actually having, any of the aforementioned merits.
A company also exists under the acronym GROM. http://www.grom.com/ Grom Associates is based in New Jersey. Coincidence? You decide.