"overlapping-overlapping-overlappers"—so funny! Keep on overlappin! It could go on for
generations, oops, I mean decades, centuries, millennia.
I wouldn't be surprised some day if they officially apply their overlapping notion to the entire period from Pentecost of 33 to Russell and down until now. I remember how when I was in, I used to hear heavies say things like "God has never been without his anointed witnesses on earth" with "since Pentecost" understood. So there this overlapping notion was all along... I think Freddie Franz got the idea from the wheat and weeds parable, "let them [both the wheat and the weeds] grow together until the harvest," Matt. 13:30. Apparently FF saw this as overlapping presences of anointed ones on the earth from Pentecost until the harvest, or modern times. I guess Splane just jumped on the notion and applied it to the generation of Matt. 24:34 when the old view of that verse was obviously no longer tenable.
These current GB guys are all originally from the great crowd, and at some time they came to believe they have been called to heaven. Since they had all been full-timers for years with some degree of prestige and rank in the org (CO, missionary, Bethelite administrator), as soon as they first partake, word quickly spreads, so they are then expected to rise in the org even more quickly. Doubtless that includes whoever is on the GB at the time, and they keep their eyes on such men to see if they're good GB material (Are they unquestionably loyal? Do they have any tendencies that might rock the boat?).
After Sanderson had started partaking for a few years and was called to Brooklyn (if that chronology is right), one normal Bethelite said that everyone expected him to be appointed to the GB after a while. It's just the way the system works. You know "holy" they are all viewed.
As to how one announces it, yes, it's that first time a person partakes of the emblems at the memorial. I know one long time elder who felt he'd been of the great crowd for years and then felt Jehovah had called him to the "little flock." He didn't tell anyone about it. He just partook at the next memorial. His wife about crapped her
pants, err, I mean undies and dress. He never spoke about it with her prior to his first partaking.
Telling people about it ahead of time would be a real no-no in Witness-think. It would cast serious doubt on its validity in they eyes of those in charge of the org. As for how one knows, that's part of the mystery of it all. Posing such a question here or elsewhere will yield no definitive answers.
I guess you or we can keep on wondering as they keep on overlapping!