You have settings for who can send you private messages: all members, friends, all models. If you just add a bookmark, you can find them again more easily but they can't PM you like they could if they were a friend, for example.
It's a little different than that. By default, PM is open for everyone to PM you. It's up to the member to go into their settings and restrict down who they want to receive PM's from. For myself, I keep it to Friends and Bookmarked models. Though, it does require some management.
Here's how I manage the lists:
Friends: Models and members I add to my friends list. These are typically models I visit regularly, and members I might have PM conversations with. Members can only add members to friends list, not bookmark them.
Bookmarks: If I find a room I want to visit, I'll add the model to the bookmark list. This way, if she wants to PM me, she is welcome to. If it's a model I enjoy, I will leave her on the list until a later time where I might become more of a regular. If it's a room I decide I won't visit again, I remove here. This way, I don't get those "Hei BB! I need tokens, cum visit meee!!!!" PM's.
I had to do this as I was getting spammed via models I'd never visited begging for tokens. Word of note on the friend's list: There is no hiding your online status from anyone who adds you to their friends list, and any member or model can add you without your knowledge/approval.
Recently, I went so far as to remove all members from friends list as I was starting to get a few of them asking me for tokens as well.
There is one key difference I noticed between model friends and bookmarked models. When you go into "Model Notifications", any models who you have in your friends list you can select their name in the drop down menu. If it's just a bookmark, you have to enter the name into the text box. I think there's some other things as well but can't recall of the top of my head.