I'm not sure if this is more than the early version of duelbits' campaign but it looks like it is.
Duelbits were accepting 115 members back then, so Rainbet breaking the record for becoming a signature campaign with the highest participants of all time.
I'm not sure if it's the most massive campaign ever, but there were also some massive campaigns in the past. As I remember, for example, Yobit, but there was no public spreadsheet, because the counting was automated. I think there was a thread with cheaters and spammers from that campaign, and there were more than 100 of them alone.