California law requires that notices to move out be put in writing. You should always require that the tenant provide a written notice that is signed by all tenants, and if you are giving them notice then you should also put it in writing and serve it personally or as is legally required.
Why is this so important? Well in most cases you will never need to legally enforce the move out notice. But let’s say that they do not move out–your legal recourse is compromised by not having proper legal notice.
Many people feel that a text message or email will suffice, but try to show that to a judge and you will have problems.
Take the extra time and effort to ensure that you have legal notice and then if a problem arises you will be a much better position.