how to get the players friends?

local Player = game:GetService('Players').LocalPlayer -- Get the Player

local PlayersFriends = Player:GetOnlineFriends() -- This is how.

But in your case, since you need to allow them to transfer to the same team, you would need this.

local Players = game:GetService(Players)

local PlayersinPlayers = Players:GetPlayers() -- get the players

local AbleTeams = {}

for i, v in pairs(PlayersinPlayers) do -- and an example of how you can do it
    if v ~= Players.LocalPlayer and Players.LocalPlayer:IsFreindsWith(v.UserId) then
        table.insert(AbleTeams, v.Team)
    end
end

--i think you can do it from here