server.lua

local ESX = nil

TriggerEvent('esx:getSharedObject', function(obj)
    ESX = obj
end)

ESX.RegisterServerCallback("esx_marker:fetchUserRank", function(source, cb)
    local player = ESX.GetPlayerFromId(source)

    if player ~= nil then
        local playerGroup = player.getGroup()

        if playerGroup ~= nil then
            cb(playerGroup)
        else
            cb("user")
        end
    else
        cb("user")
    end
end)