You were close.

You are close. You have choosing a random player 50% done. All you have to do now is index the table of players.

What you were doing was generating a number from 1 to the number of players in the game, but not doing anything with it. The table won't index itself on its own.

local function choosePlayers()
    local users = game.Players:GetPlayers()
    local user1 = users[math.random(1, #users)] -- index the table returned by GetPlayers
    local user2 = users[math.random(1, #users)]

    if user1 == user2 then -- the same player might get selected twice.
        while user1 == user2 do 
            -- This loop is entered only if the above is true. 
            user2 = users[math.random(1, #users)]
        end
    end
    return user1, user2 -- return the selected users for convenience 
end

while true do
    wait(3)
    local user1, user2 = choosePlayers()
    print(user1, user2)
end

Hopefully this answered your question and if it did, then don't forget to hit that "Accept Answer" button. If you have any more questions then feel free to leave them down in the comments below.

This script shouldn't be too much work at all to make, but for a complete beginner it may look complicated. The first thing that we should do is get out of that local script. You can not teleport players from a local script and defining a single player is completely useless. Now that we have that out of the way, we can begin making this happen. we'll start with making a way to tell if a player has been chosen:

game.Players.PlayerAdded:Connect(function(plr) -- run the code inside every time a player joins
    local Stats = Instance.new("Folder", plr) -- make a folder
        Stats.Name = "Stats" -- change the folder's name to "Stats" for later reference
        Stats.Parent = plr -- make the parent of the folder the player
    local HasPlayed = Instance.new("BoolValue", Stats)
        HasPlayed.Name = "HasPlayed"
        HasPlayed.Parent = Stats
end)

Now that we have a way to check if a player has been in already, we can start choosing them:

-- I would put this in a different script for organization, but you don't need to


local HasNotPlayed = {} -- This table will be used to choose which players can be selected.

while wait(3) do -- I'm assuming you wanted to do this once every 3 seconds for some reason?
local users = game.Players:GetPlayers()
    for i, user in pairs(users) do
        if not user.Stats.HasPlayed then
            table.insert(HasNotPlayed, user) -- puts every player  that hasn't played inside the table
        end
    end
    for i, user in pairs(users) do
        if user.TeamColor == --[[whatever the team colors for both of the fighters are.]] then
            user.TeamColor =  --[[whatever the spectator team color is]]
            -- the above code checks if the players were fighting, and if they were, puts       them back to the spectator team.
        end
    end

    local Chosen = math.random(1, #users)
    local ChosenTwo = math.random(1, #users)
    for i, user in pairs(HasNotPlayed) do
        if i == Chosen then
            user.TeamColor = -- Team color of the first fighters team
            table.remove(HasNotPlayed, i) -- removes the chosen player from the table.
            -- Teleport the player to the first spawn 
        end
        if i == ChosenTwo then
            user.TeamColor = -- Team color of the second fighters team
            table.remove(HasNotPlayed, i)
            -- Teleport the player to the second spawn
        end
    end
    local HasPlayed = 0
    for i, user in pairs(users) do
        if user.Stats.HasPlayed then
            local HasPlayed = HasPlayed + 1
        end
    end
    if HasPlayed == #users then
        for i, user in pairs(users) do
            if not user.Stats.HasPlayed then
                table.insert(HasNotPlayed, user) 
            end
        end
    end
end

Tell me if I missed anything in the comments, or if an error occurs.

Hope I helped!

-Cmgtotalyawesome