IP is sometimes used as an abbreviation for "in-patient" as in one that is a patient in a hospital that does not go home at night.
This term is often used to refer to staying at a psychiatric or mental hospital, a place that no one actually wants to go, but sometimes agree to go until they are more "stable." One can also be forced into IP if they are a danger to themself or others or unable to care for themselves. The alternative is "out-patient" in which one goes to group and other forms of therapy usually for 5-8 hours and then goes home.
Used online more often than in verbal conversation.