I just want to know if it is legal for a landlord to allow an additional person to sign a lease months after the original lease has been signed? Also is there a maximum number of tenants that can be signed on a lease? I thought that only two people were allowed per bedroom, but that might have just been a specific apartment rule that I read somewhere. This is all for MN, I just haven't been able to find any good answers to these questions so far, so any help would be greatly appreciated. Thanks!

OK I still haven't gotten any help, but I was looking up the MN Landlord Tenant law and it says this about ending the lease: Tenants with a definite term lease have to pay for the entire term no matter when they leave, unless the landlord agrees to accept new tenants who would take over the remaining payments.
In my case this is what happened; I left my one bedroom apartment that I was sharing with my ex, and I was still going to pay my half of the rent through the term of the lease, but he now has a roommate that moved in with him. I just found out that the roommate was added on to the lease without my knowledge and that they were still expecting me to pay half the rent. To me it doesn't seem legal that they can just add this other guy onto a lease and it seems to me that the original lease I signed with my ex should have been broken and then my ex and his roommate should have signed a new lease.

If two people sign a lease and one of the two leave before the lease period ends, the person leaving is still responsible for the payment each month unless the rental property is re-rented or the lease ends. In this case, a new person has taken the place of the former tenant.

Most leases read that each tenant is responsible for the full amount of the rent, not just their portion. If I were you, I would stop paying your portion of the rent, knowing that someone has already taken your place. I would also go to the rental office and ask to see the copy of your most current lease. You may be surprised to see that your name is no longer on the lease. If it is still on the lease, explain what has happened and ask that it be removed or ended and a new lease signed by the current tenants. If you have a security deposit with the landlord, be sure to solve that problem while you are in the office as well. Let us know what happens.

Thank you so much for your answer, I already sent my May rent check in yesterday though along with a request for them to send me a copy of the lease with the new person signed on it. My ex's grandma manages the apartments, so I've got to be sure before I go in there and make a big mess that what I'm doing is right. So if I call down there and find out that I am still on the lease and I request them to take me off the lease and she tells me that since I've signed a lease I'm stuck on it till the end what can I say?

Call the bank and put a stop on the check. Landlord advocate is correct.

Well all right I just called the apartments and talked to them. Basically what I was told is that I am still on the lease and still responsible for my part of the rent (or all of the rent if the guys don't pay) and this new guy will just be added on and will just be responsible for his part of the rent. I got a number for the actual apartment manager and he just said "Oh thats a tough one, let me call and get the details and then call you back" yeah I'm sure. So is there anything I can do?

I would go on the offensive in this case.

I would send a letter to the landlord by certified mail. In the letter I would say that I became aware that a new tenant was added to my lease without notice and WITHOUT MY CONSENT and therefore the landlord and the other tenants have violated the lease and it is null and void with regard to me. Then I would say that as a result of this violation of the lease I am no longer a tenant and no longer responsible for payment of rent. I would end with a demand that my share of the security deposit be returned to me within 10 days.