l think it just means guy, but what exactly does this mean?

Not sure where the word originates, but basically it boils down to one who surfs. A grommet is a young person who surfs.

Yup, grom is short for grommet. basically meaning a young surfer.

From surfline: ''Among the most important terms in surfspeak, grommet generally refers to a young surfer, one often, but not always, insolent or smarmy toward his elders. Typically sporting sun-bleached hair, sun-split lips, baggy trunks & a goofy grin, the grommet has for decades been the subject of cruel & unusual punishments. Exact history of the term is hazy, but accepted theory has it being a bastardization of its '50s California forerunner ''gremmie,'' which itself was a variation of gremlin. And a gremlin is what little surf kids often look liked with sun blisters, dirty feet & salty skin.

basically a kid or a young surfer