What it is trying to say is as long as all team members are using the same codex to make their army lists and there are no allies from other codex' that team may use special characters.
Special characters can be used by that team as long as they don't confilt with each other (such as bring two C-tan) which isn't allowed by the codex.
Special characters can only be used once per team, but every team member could bring a different special character as long as they don't confict by the codex rules. So up to 4 seperate special characters could be used- one by each team member.