You're right that the most widespread usage of «быть в теме» is to mean “to be up on”. But words tend to stretch meanings through time, they tend to drift away further and further to tangential meanings. So people started to use «быть в теме» at some point as a euphemism or slang for “to belong to group X” or “to a have an interest in X”, especially in subcultures.
To me it makes sense, because to be up on something quite often means to know something exclusive that other people don't know, which in turns quite often entails to be part of some exclusive group, or at least be related to that group somehow. That would explain how gradually the meaning generalized to mean from knowing latest news to belonging to a group in general.
In LGBT slang «быть в теме» is widely used to mean “to be a lesbian”, “to be part of lesbian subculture”. As people tend to flex words grammatically for slang purposes, the noun «тема» at some point started to mean the lesbian subculture itself. On Russian social network VKontakte there are plenty of groups and pages catered for lesbians with various variations of the word «тема», sometimes capitalized. Quick googling shows that people use the phrase «быть в теме» to denote members of BDSM, tickling, and even hipster community. I'd say it would be used even wider by various subcultures, if it weren't associated with lesbians so much. (It seems intuitively correct that in a society with heavily entrenched homophobia people would tend to not use slang or even behavior that is associated with LGBT, for some psychological reasons I don't really understand, but I never read such studies).
I would be grateful to any linguist that can explain and elaborate this phenomenon of drifting and widening of meanings of words, which produces slang and euphemisms, so that this answer could be improved.
