[Urtgard]

local f = CreateFrame("Button","TabParty2",nil,"SecureActionButtonTemplate")
f:SetAttribute("type","target")
SecureHandlerWrapScript(f,"OnClick",f,[[
    local unit
    for i=1,5,1 do
        TabPartyIndex = ((TabPartyIndex or 1) - 2)%5 + 1
        if TabPartyIndex == 5 then
            unit = "player"
        else
            unit = "party"..TabPartyIndex
        end
        if UnitExists(unit) then
            self:SetAttribute("unit",unit)
            break
        end
    end
]])

i=5,1,-1 or i=1,5,1 doesn't matter in this case.

The real counting is:

TabPartyIndex = (TabPartyIndex or 0)%5 + 1