States which have participated in one or more of the earlier Sessions of the Conference may become Members of the Hague Conference by accepting its Statute. Other States must be admitted by vote: admission is decided upon by a majority of Member States voting on a proposal made by one or several of them. (Statute, Art. 2)
Non-Member States can also become Parties to a Hague Convention, provided it has already entered into force. See for more details the FAQ section on this website (question [o]).