Sorry, I realize I didn't actually answer your question about roles...
To manage user roles and find out what roles a user is in, use static methods on the RoleManager class, which is in the Checkbox.Security namespace.
Code:
//List names of roles a user is in
public static string[] GetRoles(IIdentity identity);
//Get the database ID of a role
public static int GetRoleID(string roleName);
//Associate a user with a role
public static void AddIdentityRole(IIdentity identity, int roleID);
//Remove the association between a user and a role
public static void RemoveIdentityRole(IIdentity identity, int roleID)