How do I add more than one role id in this script?

You would need to put an "or" after the first role id. This is how it should look:

local Pants = script.Pants
local Shirt = script.Shirt

local GroupID = 6763603 

script.Parent.ClickDetector.MouseClick:connect(function(Player)
    if not Player:IsInGroup(GroupID) then return end
    if Player:GetRankInGroup(GroupID) > 255 or >123 then return end --Replace the second number with you number. 
    local Character = Player.Character
    if Character == nil then return end

    -- Get new shirt
    local CharacterShirt = Character:findFirstChild("Shirt")
    if CharacterShirt then CharacterShirt:Destroy() end
    Shirt:Clone().Parent = Character

    -- Get new pants
    local CharacterPants = Character:findFirstChild("Pants")
    if CharacterPants then CharacterPants:Destroy() end
    Pants:Clone().Parent = Character
end)