The United States formally joined a new World Bank agency to promote foreign investment in capital-hungry developing nations.

The action brings the Multilateral Investment Guaranty Agency into force.The agency, whose authorized capital slightly exceeds $1 billion, will insure foreign investors in developing countries against a variety of political risks.

So far, 29 nations - nine industrial and 20 developing - have ratified the convention creating the agency, which also will offer investment-related policy advice to the developing countries.

The contributions by the 29 countries joining the agency so far will total about $580 million. The United States, the biggest single shareholder, is providing $220 million, although only 10 percent is in cash.

More than 30 other nations have signed the agency's convention, indicating that they will soon join the agency.

The agency, which will cover investments only in member nations, will offer insurance against war, civil strife, currency non-convertibility, expropriation and contract repudiation by host governments.

It will supplement existing government insurance programs, especially in larger development projects, and is expected to coordinate insurance in projects involving investors from a number of nations.

Barber Conable, the World Bank president who will chair the agency's executive board, plans to call a June 8 meeting here of the agency's member nations to decide on its by-laws and other matters.

The agency's president may be a Japanese - Yoshio Terasawa, a senior executive with Nomura Securities in Tokyo. Mr. Conable reportedly has sought a Japanese to head the agency.

The United States meanwhile will seek to write several strictures into the agency's operating procedures. It will propose barring insurance in countries that fail to adopt internationally recognized working standards and that impose export and local purchase requirements on foreign investors.

It also will seek to have the agency turn down insurance for projects that would add to the world oversupply of a commodity.

Those rules were urged by the U.S. Congress when it authorized U.S. participation in the agency. If the agency's other members refuse to accept them, the U.S. executive director at the agency will be required by U.S. law to vote against insurance in such instances.