Rabbits are social animals, and can live happily together provided consideration is given to their compatibility.
When introducing adult rabbits they should be given time to get used to each other before having direct physical contact, as serious fighting may occur. This is best done by putting them into individual pens separated only by a wire mesh for a few weeks to allow them to get used to each other.
They can then be put together (under supervision!), ideally in a new and different environment to reduce any territorial behaviour between them.
After some initial chasing around they usually settle and can go back into their permanent home. Rabbits from the same litter can usually be kept together with no problems, although sometimes fighting will start once they reach sexual maturity.
To prevent rabbits fighting, or if fighting starts between littermates, then neutering is the best option to prevent this. Of course, if a male rabbit is to be kept with one or more females, then he should also be neutered to prevent an unwanted population explosion! It is possible to keep rabbits with guinea pigs as companions, but care should be taken as rabbits can harm guinea pigs quite easily if they don't get on.