In order to have a proper Capture-the-Flag game, we first need to have teams. There are several things that need to be associated with a given team:
Flags (and the flag color)
Players (and the player skins)
Spawn points
Bullets (if you want to avoid same-team damage)
Potentially map decorations/materials
The simplest way to associate each of these elements with a team is to just add a property with a simple value such as R or B to represent Red or Blue (or some other team name). A more advanced approach could be to create a Team class that holds references to everything that belongs to that team, since that could offer optimizations such as limiting the number of collision checks that need to be performed. If you do that, however, make sure you remove all the appropriate references from the Team container when removing something (such as a bullet) from the world in order to avoid memory leaks.
Next, we need to modify our map to add flags for the Red and Blue teams, which we'll represent...