[trianglesphere]

It's (as far as I can tell) not open to the public in terms of hosting so they don't need to be Sybil resistant.

This doesn't seem to be much more than a fancy way of mixing randomness together (which produces valid randomness if just one source was truly random) and then using BLS signatures on top of that. BLS signatures let you verify the nodes that were involved, and it seems like with how they treshold, they're trying to prevent attacks based on not revealling (ie. the randomness is the aggregated threshold signature of the random data they mixed together).

[jeffrallen]

I run one of the drand nodes. Yes, the membership is limited. If someone wanted to join they can by just asking and committing to run a node for the long term. It is not a permissionless system, and so Sybill attacks are not in the threat model.