Visit the Member's Profile by either clicking on the profile button within one of their posts or by clicking on their User Name anywhere in the forum.  This will bring you to their Profile Summary Page.  Click on the "Modify" button in the top right corner on that page.  Enter your password, and click Go, then click on Admin Edits.  You can add the Member to a Member Group in two different areas.

First, you will see a drop-down selection box labeled "Position."  This is the main Member Group of the Member.  This is the first Member Group (and possibly the only, depending on your forum settings) that will display in all of that User's posts.  This is the Member Group that determines how many stars and what type of stars display underneath the User's displayed name.  Select only one Group in this box.

Just below that, you will see a list box labeled Member Groups. You can assign a User to any or all of these Member Groups.  For each Member Group you choose in this box, the User will inherit whatever permissions you have assigned to those Groups.