To assign a contact with a persona, you will have to setup rules first, then let the calculation figure out the point in order to assign the correct persona to the contact. The rules can be set based on Attribute, Activity and Macro.
In some cases you don't need persona in order to do content targeting (only Recommend documents to personas requires that feature). When you enable content personalization, you can add personalization variant to webpart/widget directly and the display condition can be based on rules like "Contact is in role" or "Current user is in role" which doesn't require persona. You can find out more here on