Their locations are random and will change from time to time, so check taverns regularly until you find all of them. When exploring the taverns of Calradia you will come across some named people who are willing to join your cause, for a price of course.
In Mount&Blade you are not the only hero. The two-way arrows indicates companions that like each other, which can help balance dislikes, increasing the effective limit. If you wish to maximize your number of companions, you can choose 2-3 from the top circle and 4-5 from the bottom circle that are not touching. Each companion dislikes the companions on either side of them (Example: Borcha dislikes Deshavi and Klethi).