This is not really a criminal question except to the extent that M could get evicted for committing an illegal act. It is not clear if M or her boyfriend actually signed the lease - what I would do is consult a landlord tenant attorney before speaking to the apartment manager to protect yourself from any liability and also ask what happens if you move out. Good luck
It sounds like M and her boyfriend are not legal tenants which is why the apartment office is telling you they must sign the lease or move out. You have to protect yourself, however, and if the cops ever decide to make a bust they are going to take everyone. You may need to threaten an action in ejectment to get them out which requires a civil lawyer. Your most immediate need is a criminal attorney who can intervene with the police for you.
Mr. Kaman is a highly respective, competent attorney. I would recommend that you listen to him since you definitely need to protect yourself.
The response above is not intended as legal advice since it’s impracticable to provide thorough, accurate advice based upon the query without additional details. It is highly recommended that one should seek advice from a criminal defense attorney licensed in your jurisdiction by setting up a confidential meeting. Moreover, this response does not constitute the creation of an attorney-client relationship since this message is not a confidential communication because it was posted on a public website, thereby publicly disclosing the information, which is another reason to setup a confidential meeting with an attorney.
What your roommates and their boyfriends are doing might be illegal or they might be qualified medical marijuana patients or their legal caregivers. Without more facts it's impossible to say. The fact that roommate L told you that roommate M is in possession of inarguably banned substances like speed and crack doesnt mean this is anything more than a false allegation. Regardless you should protect yourself legally and move out if you can. If your apartment company won't let you get out of the lease hire an attorney who handles landlord/tenant disputes. Hopefully a sufficiently aggressive attorney letter which mentions the illegality and uninhabitability of your living situation will make the apartment change its tune. Good luck.