Validity of a Drug Possession Charge in a Shared Space

My question involves criminal law for the state of: California

Few weeks back my friend and I were in a car down by the coast, after having smoked some marijuana a few hours previously, when we were confronted by local PD.

We were both cited for the marijuana, even though the marijuana belonged to my friend, due to the fact that the plant was in the car and not on either of our persons.

From what I've read on the internet about similar situations, you can be charged for knowledge of marijuana in a jointly owned space even if it didn't belong to you, but I haven't been able to really find out if it applies to spaces in which you are a guest, I.E. the shared space of a friends car, which I would hold no deed of ownership to. I'm fairly certain it doesn't make a difference, but I'd really appreciate some clarification to see if the citation is worth contesting.

Thanks

Ownership isn't so much the question. Being a guest or not isn't the question. Google the term "constructive possession". THAT is where they've likely got you. Anyone in the car who had constructive possession could have been charged. That's why it doesn't pay to do drugs, or hang out with people who do - because you risk charges against YOURSELF, even if you don't have them on your person.