Since committees must have 128 validators, what happens to the most recent validators which are beyond the last 128 group?
Since they cannot be included in committees, are they just queued until there are 128 validators so that a committee could be formed?
The number 128 is not a must but rather a target. In the [specs] 1 it is actually defined as the TARGET_COMMITTEE_SIZE.
They write:
For the safety of committees, TARGET_COMMITTEE_SIZE exceeds the recommended minimum committee size of 111; with sufficient active validators (at least SLOTS_PER_EPOCH * TARGET_COMMITTEE_SIZE), the shuffling algorithm ensures committee sizes of at least TARGET_COMMITTEE_SIZE.
Next, to determine the actual size of the committee, they use this target when possible, as shown in the function determining the number of committees:
def get_committee_count_per_slot(state: BeaconState, epoch: Epoch) -> uint64:
"""
Return the number of committees in each slot for the given ``epoch``.
"""
return max(uint64(1), min(
MAX_COMMITTEES_PER_SLOT,
uint64(len(get_active_validator_indices(state, epoch))) // SLOTS_PER_EPOCH // TARGET_COMMITTEE_SIZE,
))
As you can see, the return function makes sure that there is at least uint64(1) committee per slot, and if it is possible to do more, they must at most be MAX_COMMITTEES_PER_SLOT which is 64 or the number of committees such that all of them have at least TARGET_COMMITTEE_SIZE member.
After that validators are pseudo-randomly affected to committees of at least TARGET_COMMITTEE_SIZE: https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/beacon-chain.md#compute_committee
