As a general rule, all adults living in an apartment should be named on the lease and should be co-responsible for payment of rent and maintenance of the unit. This means that each of you is responsible for making sure the entire rent is paid each month and that other obligations under the lease (maintenance, payment for damages, etc.) are everyone's concern. This also means that you are all protected under the terms of the lease.
If a roommate is moving in during the course of an existing lease, the landlord may run a credit check on the new roommate, and his or her name should be added to the lease. If a security deposit is required, the new roommate should pay his or her share.
It is also possible that the landlord will require a new lease at a higher rate if a new roommate moves in.
Unless you are covered by rent control or the increase is obviously an attempt to prevent the additional roommate, he is entirely within his rights to do this.
Payment of Rent
The roommates must agree amongst themselves who is responsible for which share of the rent, whether they will pay it to one roommate who pays the landlord or if they will pay it individually. This agreement is outside the lease, and the landlord isn't bound by it nor can he enforce it.
As with the rent payments, roommates need to agree amongst themselves how the utilities will be paid each month, whose name the utilities will be in. In my previous roommate situations, I have found it useful to split up the utilities, so each roommate has one or more in his or her name. Each roommate is then responsible for paying a share of the utilities each month to the roommate who receives the bill.
Deciding before moving in who will get which bedroom and how common rooms will be used may prevent a lot of problems later on. It is also useful to set out how private the private spaces are (e.g. does roommate A mind if the others go into his room to use his computer?).
You and your roommates are friends, you've partied together, you like each other. However, noise is one of the things that can drive a wedge into that friendship. Decide before you move in when the "quiet hours" will be. Does everyone have to turn the TV or stereo down after 10 on week nights? Are parties allowed on weekends, and if so, what arrangements must be made ahead of time with other roommates?
Come to some agreement about overnight guests, both of the romantic type and those who expect to sleep on the couch. If you only have one living room couch, it will be kind of difficult for two roommates' sisters to sleep on it, so how do you reserve it? And what about those boyfriends/girlfriends who all but move in?
Are you going to share food expenses and eat your meals together, or is it every man/woman for him/her self? How will you share responsibilities for cooking and shopping? How will you know whose food is whose?
How clean will you keep the apartment? Who is responsible for which household chores? When should those chores be done?
What happens if one roommate needs to leave before the lease ends? How much notice should he or she give? Does he or she need to find a replacement roommate (with the approval of the other roommates and the landlord)?
Your landlord has no obligation to enforce any agreement made between roommates. There are mediation services which can be used to mediate legal disputes between roommates. Decide ahead of time what you'll do if you come to an impasse.
Get it in Writing
NOLO provides this sample roommate agreement which you can use as a template for making your written agreement with your new roommates. It should clearly spell out the "rules" governing your co-tenancy and be signed by each roommate. If you want, you can even take it to a notary public and have it notarized, though that's not really necessary.