This might be a silly question, but I do not currently have access to all my books. Can a horde be engaged in a grapple? Or, rather, can a horde engage a player in a grapple? I know a grapple has a limit on the number of participants, but in a lot of ways a horde is considered a single entity from an abstract rules perspective. Anyway, any thoughts on the matter (or even an outright reference if there is one, either way) would be greatly appreciated.

From a rules perspective I can see it getting messy, but on the other hand, I can see a mob of zombies trying to haul someone to the ground via grapple before following up with more vicious attacks.

I personnally don't see any reason why a horde cannot grapple a space marine to the ground. There aren't any specific rules concerning it though.

I would do the following though.

"A horde may only attempt to grapple a target once a turn. For every 20 magnitude a horde may grapple an additional target in range (1-39= 1 target, 40-59= 2 targets). It makes a single attack roll with +10 to hit per degree of magnitude. The horde also gains a bonus of +10 to the opposed strength roll for each degree of magnitude they have. If the target is grappled no other enemies may strike the target in hand to hand and any shooting attack against the target also automatically hits the horde and vice versa (due to the mass of bodies in the way). (Next bit is optional, may be too powerful) Additionally they gain an extra degree of success per degree of magnitude (perhaps limiting this to 2-3)."
This represents the numerous hands and weight of men/creatures pulling the marine down. Even with such modifiers its still not certain of success, as space marines are still massively strong. A 30 mag horde of humans with the above rules would roll strength on 60, While the space marine is still likely rolling on 60+ with auto-2 DoS if passed. If the horde failed one can assume the space marine just stood there un-moving while the throng attempted to pull him down, or some other interesting visual.

Space Marines wouldn't be able to grapple a horde, though. Its just not possible to simultaneously grapple 20-100+ guys. So if a horde managed to grapple a space marine the marines only options available to the marine should be to Break Free, or Slip Free.

Strangely, that is the consensus I reached with one of my players that I bounce mechanics ideas off of. I appreciate the feedback, and still welcome any additional interpretations as well.

Since breaking free entitles one to take a half-action, I can see a Marine shaking off a horde of plague zombies and hacking down some of their number before being pulled down again... perhaps to the ultimate end of his duty, or perhaps to rise again and strike down yet more of the foul ghouls in the name of the Emperor.