Filter resource have a "sharedUsers" block that has standard pagination type attributes:

sharedUsers: {
  size: N,
  start-index: A,
  stop-index: B,
  max-count: C,
  users: [ .... ]
}

The problem is that size=0 when you first hit the resource and only shows the real size once you send in &expand=sharedUsers.

The reason for this is that we are trying to avoid doing work (i.e. database queries or loading all the data into memory). To do this in a good way will require changes to the UserService (and possibly GroupService?) to expose appropriate count methods.