Picked Teams

Setup 8 command blocks all connected to each other, that all run the command "/team create", alternatively create 8 empty teams. Setup another 8 command blocks that set the colours of the teams to the colours that you've built them as, so "/scoreboard team option UHC<1-8> color <color>"

In the middle of where the players pick there is 1 command block, that when triggered, broadcasts that the player is picking. "/bc @p is now choosing" would do the trick.

Above the area, or in a place only staff can access nearby, have a command block with a lever on top that teleports a random player not on a team to the coordinates of the picking area. I don't remember the wording for players not on a team, I'd have to check that with someone else.

The player has (normally) 8 teams to pick from, there should be a pressure plate at each of the available teams. That plate should trigger red stone to 3 different command blocks, one of them delayed with a repeater. The first of these will run "/bc @p has join the <color> team". The second will teleport or warp the player to the cage/area of their team, this one should be the delayed one. The third command block adds the player to the team, so "/team set @p <number>"

Once a team is full you should block off the option to choose said team and announce that is no longer available/full.