After the multi-lateral treaty’s scope and priorities are negotiated by the few countries invited to participate in the early discussions, ACTA’s text will be “locked” and other countries who are later “invited” to sign-on to the pact will not be able to re-negotiate its terms. It is claimed that signing-on to the trade agreement will be “voluntary”, but few countries will have the muscle to refuse an “invitation” to join, once the rules have been set by the select few conducting the negotiations.
The US is negotiating ACTA through the Office of the US Trade Representative (USTR), an office within the Bush Administration that has concluded more than 10 “free trade” agreements in recent years, all of which require both the US and the other country to increase intellectual property rights enforcement measures beyond the international legal norms in the WTO-TRIPS Agreement

Proposed US ACTA plurilateral intellectual property trade agreement (2007) – WikiLeaks