It is not illegal to be with someone who has a warrant for arrest. You also have no obligation to actively report them or turn them in. It IS illegal to do anything to help them avoid arrest, which can include, for example, letting them stay in your home if you believe the police are looking for them (since your friend may then be hiding from the police in your house, and if you believe the police are actively looking for them, you are knowingly letting them hide in your home); driving them away from some location where the police were coming for them; answering the phone for them and saying they are not around when they are, if the caller was the police; etc. So be very careful in what you do while with your friend; anything that could be taken as helping them avoid arrest could get you in trouble.