Well, in terms of human interest, perhaps it is that growing participation means that real people and the GP become part of each other's lives in the sociohistorical world of the former! In "theoretical" terms, however, the most important concept is the Vygotskyan concept of "mediation": People experience the sensible world not directly, but as mediated by 1) symbols; 2) other tools/artifacts. We'll emphasize (1) here: We live not by rectangular horizontal surfaces supported on tubular objects at each corner. We live by tables! And a table is not a stol (to use an example from Russia). A table is one of countless pieces of life that make up my home languacultural world. A kitchen table is too. So is a group playing cards at it.
Since the world we experience is thus mediated by sociohistorical artifacts, ( including (1) tables, (2)the word "table" with it's phonetic substance and conceptual role, (3) people playing cards, (4) the phrase "people playing cards", etc. x 10 ^≈100), which are both socially inherited and dynamically constructed in situations (situations themselves being socially constructed mediational means) we call the meditational means "the pieces of life" (emphasizing the table) and the "story-construction pieces" (emphasizing the word "table"). With massive assistance from the auditory story-construction pieces, in early life we come to live by the sensible pieces of life, as stories--right as the experience goes by the experiencer and becomes part of his/her experience.
From this understanding that human life/experience/thought is mediated comes the GPA concepts such as languacultural worlds, home worlds, host worlds, "they stories," identity in the separate worlds, participation, growing participation itself, etc. As brand new growing participators, we function by our home-world story-construction pieces (we can't function without someone's meditational means) as we take on our very first host-world practices, for example host-world phonetic words (which are practices) with home-world conceptual roles (also practices). And on we grow, becoming ever more familiar with the host world as mediated by host words (sound and conceptual role), etc. (Rather than "conceptual role" I would like to say "discursive roles" but not as many people understand "conceptual" as understand "discursive"!)
So that was the main answer I was hoping for: Mediation. Oh well. If you put people first, then you're right and I'm wrong. But remember that a man is not a muzhchina. Over your years in Russia, the role of muzhchina's in your experience moves from that of men (or whatever your home-world symbol is) to that of muzhchina's (well, of muzhchin). But if your main thing is just about loving Russians, so expressed with our home-world word-analogue, then bravo.