Sorry, I'm not sure if this is specifically referring to Chainmail or early D&D. If the latter, this is explicitly not how Gygax (and Arneson) ran their campaigns back in those days, though. They had groups of people playing that fluctuated in the 30-50 player range, and people often had multiple characters specifically because they frequently did not have the same people at the table each session. They were in the same shard, persistent world, but there were many different parties, and they all decided on their own goals. These often conflicted - adversarial interactions between groups were things that happened! B2 - Keep on the Borderlands - even includes a lot of details around how the DM should handle such situations, how the players can protect their treasure from other players, etc.