If your landlord isn't evicting you over the issue, then the lease remains valid and you remain obligated to your landlord.
If you believe your rent split should be renegotiated based upon the presence of the new roommates, talk to your roommates about how rent should be split.Quoting Happy
If you don't like what your roommate is doing, you can sit down with him and talk about the problems you have with what he is doing and see if you can reach a mutually acceptable resolution.Quoting Happy
Stay until your lease is up, move and keep paying rent until the lease ends, or move and stop paying rent and see if you end up being sued by your roommate or named in an eviction action for nonpayment by your landlord. You can see if your landlord will release you from the lease, although even if he does that won't relieve you form your obligations to your roommate.Quoting Happy

