Prior to being introduced, all rabbits should have a health checkup as well as be fixed. This way you can prevent an outbreak of anything contagious. Until that time, new rabbits should not be allowed into the communal area, or be given access to the other rabbits.
What is your end goal? Do you want to have one massive, bonded group? Have you ever bonded rabbits before? If the rabbits are truly bonded, they should be left together at night. Bonded rabbits should never be separated, as this creates a lot of stress. It’s important to consider how many rabbits you wish to keep.
How many rabbits are currently part of your household? I would suggest adding rabbits one-by-one. This makes the quarantine of the new bun easier and allows you to get a better feel for personality. During bonding, you can be sure the friendship is solid and that they will be less likely to have any disputes later on from a rushed bonding process. Time is important, and you must be willing to be patient with all involved. Neutral territory is an absolute must. When introducing and attempting to bond these rabbits, they must meet and interact on a territory that neither one of them have claimed.
One litter box will likely be overwhelmed and become filthy too quickly. I’d suggest multiple litter boxes: the litter won’t become as saturated as quickly (helps prevent urine scald) and if one bunny decides to lounge in the litter, there are other places to go.