Must congress accept a member of he or she meets the basic tests age, citizens and residency

No - they must be voted in. However, they do qualify for candidacy.

I think On December 25, 1991, he resigned. By January of 1992, by popular demand, theSoviet Union ceased to exist. In its place, a new entity was formed. It was called the “Commonwealth of Independent Republics,” and was composed of most of the independent countries of the former Soviet Union.