I am thinking about group of casters casting one spell as a group. Group of mages with same level of power or some powerful sorcerer and his apprentices. This is quite popular trope. However, I am not thinking about some ritual only for the sake of the plot. I am thinking about regular in-combat casting.
Are there rules for this? For example, to allow to increase spell caster level depending on caster levels of group casting it?
There's a feat for that.
The Complete Arcane feat Cooperative Spell (76-7) allows two or more casters that possess the feat—which has as a prerequisite any other metamagic feat—and that are adjacent to each other (or, in the case of a large group, as long as each of them is adjacent to at least two other cooperating casters) to cast the same spell simultaneously to create one spell that uses the highest-level caster as its base but then has its saving throw DC and caster level checks to beat spell resistance increased based on the number of participating casters.
Basically, fifty level 1 wizards can all cast charm person at once to produce a lone charm person effect that has a save DC in the mid-60s and pretty much beats anything's spell resistance.
The main problem is that the group needs to prepare the same spells, so they are bound to the weakest casters of their group—and with just a few participants, the DC increase isn't likely worth just forcing the subject to roll a saving throw multiple times. Still, in the right hands, the feat can be pretty strong.
The Incantatrix prestige class (Player's Guide to Faerun) can ready actions to apply metamagic feats to spells cast by others, allies included, a bunch of times per day.
While the Incantatrix is not technically casting anything, she is sure cooperating to the casting of spells.
