Okay I followed the example with 28, but let’s say 32.
I think key part is one node wouldn’t need to move, though.
The 2nd node that manages to join a group would stay there.
How long would it take a random attacker node to find the group in which the first “leader” node lives? If there are 200 groups, it’d take it around 100 tries, so 500 minutes assuming 5 mins per try. But each would be trying independently, so the slowest would join in 5 * 200 groups = 1,000 minutes, no?
Maybe longer, if the group is “full”, in that case they’d have to wait longer (Tonda’s argument). But you don’t really wait - you immediately get sent to the first group which isn’t “full”, so they’d go through groups pretty fast.
If 10% of nodes go offline per day (even just briefly), you could gain 2 new memberships in the group per day.