"Well you finally made it," Washington attorney Thomas Austern teased Stanley Marcus Saturday night.

"For a while there, I never thought we would," admitted Marcus.

They were referring, of course, to the more than five-year lapse since Neiman-Marcus, the Dallas-based mecca of luxury and quality merchandise, first indicated they would have a store here.Neiman-Marcus opens for business today in the Mazz Galcrie in Friendship Heights.

Stores don't just open their doors in Washington. There is always some accompanying fanfare in the name of a worthy benefit, whether the store has simply spiffied up, added a branch, or made its debut.

Saturday night's Neiman-Marcus preview party, a benefit for the National Symphony Orchestra that will put over $65,000 into that group's coffers, was a winner, as store parties go.

Guests were greeted at the door by an NSO group playing such rousers as "Vanished Army" and "Prelude to Revelry." Orchestra groups played elsewhere in the store and six bars and several buffets were set up on each of th three floors.

It was a party short on the usual Washington glamour guests but spotlighted instead the royal family of retailing - the Marcus family was out in force, including Mrs. Herbert Marcus, 95, who is still listed as Vice president in charge of horticulture. She has missed only one of the branch store openings (Chicago).

She strode in on the arm of her son, Stanley, settling in a chair in the accessories department and holding forth with relatives and friends. She didn't have the chance to check the greenery in the store but warned that when she did "I'll be very critical." She ten dispatched a store executive to check the bromeliands she heard were upstairs.

It wasn't just the sky-high tags on some of the luxury items that reminded guests that this is a Texas based store. A rhinestone cowboy mime held cards giving directions to the bar. There were corny fogs, a Texas State Fair specialty, and a desert of trifle served from clay flower pots (with flowers) with a trowel and doused with melba sauce that poured from miniature oil wells set up in the desert display.

And 42 cases of Lone Stay Long Neck Beer shipped in from Dallas for the benefit had been polished off by the press the night before. A bockup supply was airfreighted in for the party on Saturday.

The party brought out only a few designers, including Charles Suppon, Sal Cesarani and Muriel Grateau, whose collection from Italy will lead off the designer show in the store Tuesday. But the store's competitors were out in force.

It took guests little time to settle down to some serious window shopping since actual purchases could not be made until today. Elise Gelman, a George Washington University student, was checking out diamond wedding bands to go with her Harry Winston diamond engagement ring. Jon Martinez from the store, seemed sure he could find something to please her within her price limit of $1,000.

Mrs. Edward K. Jones had slipped a black broadtail coat over her knit print dress but decided that black wasn't right for her complexion.

And Mrs. John Brademas spotted a ceramic pot with a spinach plant which she thought would make a per feet house gift and expected to be first in line to get it this morning. "Then I'll check the sables," she said teasingly to her husband. Rep. Brademas (D-Ind.), who seemed net to hear her.

Oil wells will be part of the docor in front of the store when it opens today. In other cities, butterflies, perfume, beer and orange juice have at one time spilled from these displays. Anthony Harriman, manager of Washington store, would not reveal what would be flowing this morning.

"It is probably money, or the blood of the taxpayers," suggested Stanley Marcus, who hadn't been brought in on the secrect. "Or maybe red tape."