How does many people doing the same ritual spell rules go?
Do you get bonuses or what?
I'm not sure I understand what you're asking, but I'll try to help anyway...
Under normal circumstances, Rituals are cast by a single magus as with other types of spells
Like Formulaic spells, however, characters can use Wizard's Communion to allow multiple wizards to participate in the casting of a spell. The main advantage of this is increasing Penetration and reducing Casting Penalties.
There is a "Ceremony" ability in several of the RoP books that does what you want - allows multiple participants to perform a big spell they may be otherwise unable to perform (or a moderate spell with good penetration, etc). If memory serves the Ceremony Ability itself is a supernatural ability, so you will usually need a virtue to get it,
If memory serves It's pretty useful for Infernalists, who suffer nasty side-effects when their spells fail!
If memory serves it's slightly confusing in that in some books it is a Major virtue, and in others a minor one...
I had just assumed that Ritual Magic has some special rules that give bonus when several people are casting the same spell, but could not find them. In Ars Magica fluff there is mentioned how before Bonisagus groups of mages made earthquakes.
Are there any house rules made for this?
Yes, but most of the ability to cast those big rituals was lost in the fall of the Empire. The closest thing Bonisagus managed to recover is Wizard's Communion, but it's not nearly as effective.
In Houses of Hermes: True Lineages, the Guernicus section has some rules for the Mercurian Rituals that were recovered by Fenicil of Guernicus in the 9th century - available now only to the senior members of the house. Basically, each ritual is treated like a separate ability that must be learnt by each participant; the casting total is equal to Stamina of leader + Sum of ability score of all participants + Aura modifier + die roll. The leader must beat a really high target number to get the spell off (78 for major rituals), and the penetration of the effect is equal to the difference between casting score and target number. Oh, and these things cost rooks and rooks of vis to cast!
Alternatively, a number of supernatural traditions from all the four realms can use the Ceremony supernatural ability to achieve a very similar effect with supernatural abilities of that tradition - essentially combining the scores of different casters in a group together to achieve one big effect. From my experience, if you do manage to get a sufficiently large group together, these magics can be devastatingly powerful.
Noble's Parma, but I believe the quote is referring to the pre-Hermetic Cult of Mercury. All of the magic employed by that tradition involved huge rituals (sometimes lasting up to a Season) involving vast numbers of casters and prodigious quantities of Vis, but were capable of correspondingly impressive feats of magic.
The Guernicus chapter of Houses of Hermes: True Lineages has a few sample Mercurian Rituals that sometimes still see use by the modern Order of Hermes, but they're not really practical for most Hermetic Magi...