None of the answers posted thus far have noted the earlier sexual meaning of grommet in English, U.S., and Australian slang. Jonathon Green, Slang Dictionary (2008) notes both that meaning but and the "surfer" meaning, but doesn't claim that one arose from the other:
grommet n.1 {S[tandard] E[nglish] grommet, a ring of rope, a washer} {late 19C–1940s} (US) 1 the vagina; thus generic for an attractive woman. 2 (Aus.) the anus. 3 sexual intercourse.
grommet n.2 {surfing use, grommet, a novice} {1990s+ (Aus.) a surfer
Eric Partridge, A Dictionary of Slang and Unconventional English, eighth edition (1984) has only the sexual meanings (not surprising given that, according to Green, the Australian surfer meaning arose in the 1990s, although instances from the mid-1980s have since been identified):
grommet. Var. spelling of grummet, coition, hence, the female sex.
...
grummet (or grommet,-it). The female pudend: low: nautical, since mid-C.19, > low gen. Ex grummet-hole, itself ex grummet, a little ring serving merely to tie gaskets (Manwayring, Seaman's Dict[ionary], 1644—cited by [Ernest W[eekley, Etymological Dictionary of Modern English (1921).—2. Hence, coition: nautical and low: since mid-C.19.—3. Hence, girls and women, regarded as sexual objects: raffish, low and joc[ular]: since ca. 1950, ?earlier. Patrick Campbell, Come Here Till I Tell You, 1960, 'two lovely bits of grummet'. Also Aus.
Tom Dalzell & Terry Victor, The New Partridge Dictionary of Slang and Unconventional English (2013) have a small entry for grom and a much expanded entry for grommet:
grom noun a beginner surfer US An abbreviation of GROMMET. [First cited instance:] Surfing, p. 43, 14 March 1990
grommet noun 1. a novice surfer, especially one with a cheeky attitude AUSTRALIA, 1981 [First two cited instances:] — Nat Young, Surfing Fundamentals, p. 127, 1985 ... — Tracks, p. 82, October 1985 2. by extension, a zealous novice in other sports US Recorded in use by skateboarders by Dan Maley, Macon [Georgia] Telegraph and News, p.9A, 18 June 1989. 3. a child; a young person AUSTRALIA [First cited instance:] Dirk Flinthart, Brotherly Love, p. 33, 1995 4. the anus UK, 1889 [Only citations are from:] —Robert S. Close, Love Me Sailor p. 4 [and p, 158 and p. 221], 1945
Robert Chapman & Barbara Kipfer, Dictionary of American Slang, third edition (1995) has this:
grommet n by 1986 An early teenage or subteen person in a surfing milieu[.] Apparently a borrowing from Australia, where it is also spelled grummit: ... an ersatz club scene for junior high-schoolers, grommets, kiddies ... —Los Angeles Times {origin unknown; grummit and grommet are found by the 1890s in the sense "vulva," where the reference is probably to various rings used in ship's rigging, but the semantic connection here is uncertain}
Several slang dictionaries associate the earlier sexual meaning of grommet with rope rings used in ship rigging, and report that this slang usage was known in the UK, the United States, and Australia. For its part, the "young surfer" meaning of grommet may have arisen in Australia by 1981 and certainly existed by 1985; it is also recorded in the United States by 1986.
I am not convinced that the surfer term grommet has any connection (beyond coincidence) to the earlier sexual term grommet. However, the sexual term was known in Australia prior to the emergence of the surfer term, and the fact that the sexual term has a fairly long history in English at least deserves notice in connection with the posted question.