Let someone in/out = to let someone in/out a room e.g. You shouldn’t let anyone in after 12pm because you never know who it might be.
Let someone in for something = to make someone involved in a difficult/unpleasant situation e.g. I don’t think you know what you are letting yourself into by accepting that job offer - there are many deadlines.
Let someone in on something = to tell someone a secret e.g. Very few people knew about the promotion but the manager let me in on it.
Let someone off = to not punish someone who has done something wrong or to punish someone lightly e.g. He should have been put in jail for life. I can’t believe they let him off with 10 years only.
Let someone off something = to allow someone not to do an unpleasant job e.g. You don’t have to clean the bathroom today, I will let you off.
Let on = to tell someone about something which was meant to be a secret e.g. He let on that he was going to propose to his girlfriend.
Let out something = to rent out a room/building to someone e.g. I’m moving in with my boyfriend so I’m letting out my old apartment.