Can the police arrest me without a warrant if I am at a friend’s house?
Sometimes. A suspect may still be entitled to call a place "home" without owning or renting the house. However, a suspect who does have his own home and stays temporarily as a guest at the home of another may not necessarily claim the host’s home as his own. Remember, the overriding idea is where do people have a reasonable expectation of privacy? One criteria may be -- where do you keep your toothbrush?
Keep in mind that the major issue is the officer's right to enter the house. If the owner of the home grants them permission to enter for any reason, then they can arrest anyone inside whom they have a right to arrest.