The Jimmy Carters of 1600 Pennsylvania Ave. are starting to show Washington just how informal official White House entertaining can be.

Even when the guests are the Chief Justice of the Supreme Court, six associate justices, the Vice President and the Attorney General, with their wives.

Last night, for instance, all were expected for what Ann Anderson, deputy press secretary to Mrs. Carter, called a "private, informal, friendly, non-working" 7 p.m. dinner that featured roast beef, mashed potatoes, green beans, watercress salad, popovers and lemon chiffon pie.

Dress was business suits for the men, invitations the telephoned variety made last Friday and accepted by everyone exept Justices Byron White and William Rehnquist, reportedly out of town on previously made engagements, and Mrs. Griffin Bell, who was still in Georgia packing for her move to Washington.

"It's an official dinner which we're doing in a very informal way," said Anderson, indicating that there may be like it ahead if the consensus is that last night's "worked well."

The Carters served no hard liquor before dinner. Instead, in line with the First Lady's decision to substitute wine for the more traditional cocktail fare at social functions, guests were offered a dry white wine with black currant liqueur, which the White House identified as Vermouth-Cassis. Served with the roast beef was a 1971 California Cabernet Sauvignon.

Anderson said the early dinner hour probably meant an equally early departure by guests. That, too, appeared to be yet another sign of Jimmy Carter's intentions to reduce "workabolism" among members of his new administration.

On Monday he told members of his Cabinet that he did not want them to overdo their official responsibilities to the deprivation of their family life.

"I don't want your familes breaking up just because you felt a loyalty to me," he said.

In a handwritten note circulated among his own West Wing staff last week, Carter gave his aides similar advice. There would be nights when there would be a crisis, "when you'll need to spend the night on a couch here," he said. But otherwise, he urged them to "watch yourselves and keep your families together . . . You'll be so much more useful to me and-to the country if you do have some recreatino, get some exercise, see your children and your spouses."