Bill Fitzgerald calls it his amnesiac door--that hidden entrance to a back room at Independence Federal Savings and Loan where each Thursday for the past seven years he has assembled Washington movers and shakers for informal luncheons.

"Nothing that goes on gets out," Fitzgerald--founder, president and S&L director--has always warned. And invitations to the luncheons of about eight or 10 are among the more coveted tickets in town.

Well, the secrecy of it all simply got to be too much for Washington--a virtual hotbed of gossip and leaks. So a few years ago, a reunion for luncheon alumni was organized. For a mere $25, it really was the only way for everyone to find out just who does go over to all those Fitzgerald lunches.

Yesterday was the reunion's fourth anniversary--as well as Fitzgerald's 50th birthday--and the crowd that crammed onto the terrace of the Capitol Hilton was a melting pot of Washington society, business and government.

For starters, there was Mayor Marion Barry, Organization of American States Secretary General Alejandro Orfila, Woodward & Lothrop chairman Edwin Hoffman, practically the entire city council, socialites Tandy Dickinson and Margaret Hodges, Hechinger's chairman John Hechinger, and Charles, the hairdresser, who doesn't seem to have a last name, unless you count The First.

Fortunately, Fitzgerald is a master at handling crowds of 600. Everybody got a bear hug and two kisses. One on each cheek. Except for the men. They got a handshake and an extra long bear hug. After a while, Fitzgerald pulled out a white handkerchief to wipe off all the coral and wine lipstick that was beginning to accumulate on his face.

The popular banker was a one-man receiving line who knew not only your name, but your tennis score, your new job, your wedding plans and your graduation date.

"Hihowyadoing, darling?" he said to one pretty blond. "Congratulations."

"For what?" inquired the blond, after kissing both cheeks. Coral lipstick.

"For graduating law school," he said.

"How did you know that, Bill?" she screeched. "Do you keep a card file on me?"

"On everyone," he smiled.

Next.

"How are you holding up?" he asked Orfila. Two bear hugs.

"Not too bad," shrugged Orfila.

Before it was all over, Barry proclaimed June 3, 1982, William Fitzgerald Day, 600 people sang "Happy Birthday" and the guest of honor poignantly thanked all his friends who "taught me what I did not know, guided me when I could not see, forgave me when I erred and loved me when I needed it."

Fitzgerald, who started the first minority-owned S&L in the city in 1969, said he got the idea for the luncheon back in 1975 "when there was a large schism between the local government and business leaders. I thought small groups should be able to communicate in off-the-record sessions.

"I usually ask at least one member of the city government, a local businessman and someone from the media . . ."

"And there's almost always one pretty woman there," added Jean Viner Bell, who does the firm's public relations. "And if Bill is riding in a cab that morning and thinks the cabdriver would be interesting, he'll invite him too."

Yesterday, after the chicken and fancy birthday were done, everyone got either a crisp pink or red carnation to take home. Fitzgerald also advised everyone to make at least one new friend before leaving. It would be safe to say that everyone did.