In Illinois, how can someone be committed to a mental institution without their consent?

Illinois has some peculiar laws in terms of institutionalization. A doctor can legally commit a patient for up to 72 hours (this is referred to as a "psych hold"), but if two family members consent (notarized affidavit w/ witnesses) AND the doctor signs (three signatures in total), then an individual can be committed involuntarily for up to 90 days.

The only other way to have one legally committed is through law enforcement. This option only applies when the person is a PRESENT danger to those around OR the person is threatening suicide (note: this is not someone who has talked about killing themselves or is just crazy acting--the person must be trying to commit suicide at that very moment or be suffering from a psychotic episode/break)

Call the local police, the Desk Phone, their regular 7 digit number & get their opinion of what can be done.

They can be taken away forcefully if they are a danger to themselves and others.

So you're asking if you have the legal authority to get your roommate committed? No, you don't.