All over the world you can meet neat young American men and women with black name tags saying who they are: Mormon missionaries. Since 1991 they have arrived in Moscow, Nizhny Novgorod, Voronezh and other Russian cities. But if President Boris Yeltsin signs the bill on religious associations overwhelmingly approved by parliament last month, they may very well be gone. The law, lobbied for by the Russian Orthodox Church and criticized by many human rights organizations, would deprive religious groups that have not been registered in Russia for at least 15 years of many legal rights. They could not own or rent property, worship in public, or do charitable work. Missionary work would be banned without an affiliation with an established Russian organization. ""We have a hard time imagining that he would sign it,"" said Donald Jarvis, president of the Mormon mission in Moscow. ""If he does, we will have a great deal of difficulties.