A GROUP of Vietnamese boat people who fear their membership of an anti-communist organisation opposed to the Hanoi Government has been exposed to Vietnamese authorities by an error of the United States mail, have appealed to the Hongkong Post Office to help stop their forcible repatriation. Thirty-nine Vietnamese who sent a letter to the California-based New Vietnam Democracy Movement and Provisional Vietnam Government in 1991, enclosing a signed list of their names and detailing the activities of the Lien Minh Chong Cong (United Anti-Communist Group) which they had formed at Whitehead camp, say they received it back three months later with the stamp ''Misdirected to Ho Chi Minh City''. However, their appeal to the United Nations High Commissioner for Refugees (UNHCR) to grant them refugee status because their action had exposed them to persecution in Vietnam was refused. Without informing the group of its view, the UNHCR decided the stamp was a forgery since it was written in English instead of Vietnamese or French, the language of international mail. Now in a last-ditch attempt to forestall the forced repatriation of Nguyen Thanh Hai, who has been moved to Chi Ma Wan camp apparently in preparation for removal in the Government's orderly departure programme, they have written to the Hongkong Post Office to have the stamp authenticated. Their move has the backing of Mr Shep Lowman of the US Catholic Conference Migration and Refugee Services, who has written to UNHCR chief of mission Mr Rob Van Leeuwen asking him to investigate the matter more deeply. ''If the letter in question was sent to Ho Chi Minh City, undoubtedly these people will face serious problems if forcibly returned,'' he wrote. The letter carried not only the stamp, but a standard Ho Chi Minh postmark on the reverse. A lawyer working with the Vietnamese said the UNHCR was wrong to assume that either was forged until the matter had been checked with the Vietnamese postal authorities.