The register went live on 25 March, just over a year after the Lobbying Act became law.
Under the Lobbying Act, organisations that conduct the business of consultant lobbying are required to join the register and declare whether they subscribe to a relevant code of conduct.
All registrants must also declare the names of the clients on whose behalf they have lobbied ministers and permanent secretaries in each quarter.
Agencies that have signed up to the register include Brands2Life, Hill+Knowlton Strategies, Westbourne Communications, Burson-Marsteller, Luther Pendragon and MWW.
Meanwhile APCO Worldwide and Ernst & Young have also joined.
Organisations can apply to join the register at any time, as long as it is before they carry out lobbying.
The register is expected to cost £616,000 in implementation and first-year running costs. A report from the Office of the Registrar of Consultant Lobbyists says the Cabinet Office expects to recoup £150,000 from registrant fees in 2015-16.
There has been a mixed reaction to the register with Lisa Nandy, Labour's shadow minister for civil society, describing it as rushed, lacking the support of the industry and a waste of money.