Jack and Jill bathroom locks
You already answered it in your question.  You need to look for privacy door locks.  Also be aware that there are privacy locks that allow you to unlock from both sides (think standard bedroom lockset that gives you a small hole or allows you to use a coin to unlock from outside).  If you just have family members any standard privacy lock is fine.  If you have strangers you need to make sure the model doesn't allow the option of unlocking from other side.