If you are not already a member, CIDOC would welcome your involvement in its work. Membership of CIDOC itself is free, but to be eligible for membership you must join ICOM, which you normally do through the National Committee for your country.
Once you are an ICOM member, you need to advise ICOM that you want to join CIDOC.
You can choose to be a voting member of only one of ICOM's International Committees. If you choose CIDOC you have to notify the ICOM Secretariat of this, on the Membership form which you can download from the ICOM web site.