When you have completed the sixth level, you can go back to the Time Factory (or Tom Tom Base). Go to the easiest level and complete all of the missions. After that, you should go to a Boss stage. Complete it. You should then switch teams. So if you are a Tom Tom, you will now be a Time Sweeper. Go to the Pause menu then go back to the team that you were on. Talk to everyone in your Base or Time Factory. At least one of them should want to join your team. From there, they will tell you what to do. Do this until you have all the Team Members.