How do I make a Team Whitelist System?

What I am trying to achieve: A GUI that allows High Ranks to whitelist certain people to teams in game manually

What I have achieved: Nothing

Haven't even started yet, I have thought of a few ideas, but they were very faint and would probably never work, I would appriciate if you could help me with this. And if you need me to expand further because I wasn't clear I will gladly do so.

Edit: I am going to elaborate - I want a GUI that allows High Ranks to whitelist certain people to teams in game manually, as said above, and along with that I would also like for the people who're whitelisted to certain team to only be able to select those teams in the team select