Can the police enter your garage area if its an open garage without walls?

My garage in front of my house is an open port where only two cars can sit. There are four podiums or poles that support the roof from the main house. We ushally drink in the garage and play non loud music. Usually when we get complaints, the 5-0s usually tell us to take it inside or outback. But just recently there was a complaint early in the morning when it was daylight and the police came into our garage area harassing us. They came asked for IDs (we were all 21 and above) and while we were telling them we was cool they argued and apprehanded 3 of my relatives while they were sitting. Yes they were responding back to what the officers were saying, but does that make sense if they come onto my property and ask for ID and not warning us to take it out back? The other two officers were cool but it was the rookie who was an asshole that slammed my sister agaonst a car while she was just answering back while sitting. No up front talking smack kinda harassment, just talking back. I just wanna know if it was trespassing in an open garage and if they shouldve warned us before taking action while everything couldve been prevented?