When it comes to dancing, President-elect Ronald Reagan will do a bit of old-fashion soft-shoe at eight invitation-only balls here on inaugural night, but the 40th president plans to skip the ninth official ball -- a disco party for his young supporters, the cochairmen of Reagan's Inaugural Committee said yesterday.

Promising an inauguration with "few frills but lots of thrills," cochairmen Robert K. Gray and Charles Z. Wick announced the sites of the Jan. 20 balls and hawked the committee's wares, including special, 90-day inaugural license tags, gold medallions and memorial books. The balls and the souvenirs will help pay for the $6 million-to-$8-million festivities.

Gray said formal "white tie, black tie option" balls will be held at the Pension Building, at the Smithsonian Institution's American History and Air and Space Museums, the Kennedy Center, and at the Shoreham Americana Hotel, the Washington Hilton and at two sites at the Washington Sheraton. A youth gala, which Gray described as a "disco ball," also will be held at the Washington Hilton, but it will not require formal dress.

Tickets to the formal balls are expected to cost $100 apiece and will be sold by invitation only, Gray said. About 40,000 invitations will be sent out, with the bulk being distributed outside the District of Columbia, Gray said.

About 4,000 tickets costing $25 apiece will be sold to the youth gala, Gray said. He did not say what age the "young supporters" will be. He also did not say why Reagan has decided not to attend that ball.

Besides those balls, the committee will host two satellite balls in the city, tentatively scheduled at the Rayburn House Office Building and at the Madison Hotel, Wick said. The committee plans to broadcast the sights and sounds from the invitation-only balls to smaller, $10-a-person balls at 100 sites throughout the country via closed circuit television, Wick explained. At the satellite balls, dancers will be able to watch the Reagans on special television screens, some as large as 40-by-60 feet, and hear the ballroom music from elaborate stereophonic equipment, Wick said. The estimated $2 million cost of broadcasting the balls is expected to be paid for by contributions from corporations and individuals, Wick said.

Wick said 32 cities, including Baltimore, have agreed to host satellite balls, which will "make the inauguration a truly shared event nationwide, not just for people who can come to Washington." Tickets to the two sattellite balls here will be sold on a first-come, first-served basis and will be limited, Wick said.

Tony Bennett, Glen Campbell and Tanya Tucker are some of the entertainers booked for the balls. The Les Brown and Harry James orchestras also will appear at two balls.

Tickets to the televised Capital Centre Gala on Jan. 19, which will be limited to 19,000 persons, will be the most expensive inaugural event, costing $50, $100 and $150. The committee plans to sell broadcast rights to one of the three major networks. Johnny Carson will be the master of ceremonies and Frank Sinatra also will appear.

The first inaugural event will be held Jan. 17, a Saturday, when various state societies on Capitol Hill plan receptions for new congressmen. The first event the Reagans will attend is a Sunday morning worship service at the National Presbyterian Center. That afternoon, there will be a governors' reception followed by three concerts at the Kennedy Center. The Reagans will attend the Kennedy Center concerts and special candlelight suppers afterward. Tickets to those events will be limited.

A reception for Nancy Reagan and Barbara Bush will be held Monday morning, followed by a reception for Vice-President-elect George Bush and, that night, the Capital Centre gala.

Inauguration day will begin with the Reagans attending a private service at St. John's Church across Lafayette Square from the White House. At noon, Reagan will be sworn in on the West Front of the Capitol, the first president ever inaugurated on that side of the Capitol. The president then will eat lunch with congressional leaders before riding down Pennsylvania Avenue to watch the planned one-hour inaugural parade.

After the parade, the participating bands will return to the Capitol's West Front to perform a free concert, which will be followed by a $700,000 fireworks display. The inaugural balls will start at 9 p.m.