How to make a team chooser?

I am making a game where the players can pick their teams, but theres a problem. All the players can just go on one team. What can I do to make the teams even and stop everyone from just joining one team.

local function countTeams( player )

    local rteam = 0
    local bteam = 0
    local neutral = 0

    for k,v in pairs(game.Players:GetChildren()) do

        if v ~= player then

            if v.Neutral then
                neutral = neutral + 1

            elseif v.TeamColor == teamRed then
                rteam = rteam + 1

            elseif v.TeamColor == teamBlue then
                bteam = bteam + 1

            else
                neutral = neutral + 1

            end
        end
    end

    return rteam, bteam, neutral

end