"The Return of the Secaucus Seven," opening today at the Outer Circle, is the latest low-budget "miracle" feature, a bargain-basement independent production which brings new faces and fresh inspiration to the always needy, sluggish Hollywood mainstream.

The most successful example of the '70s was George Lucas' "American Graffiti'; the most recent was Martin Brest's "Hot Tomorrows." A delightful addition to this indispensable tradition, "Secaucus Seven' also suggests that the fine young novelist and short-story writer John Sayles has a second major career ahead of him as a screenwriter and director.

A witty affectionate and thoroughly modern comedy of manners about a group of former campus radicals slouching reluctantly but inevitably toward maturity (symbolized, naturally, by the age of 30) in the course of a summer weekend reunion in New Hampshire, the movie has already earned Sayles the 1980 best screenplay award from the Los Angeles Film Critics. Capital choice.

Nothing particularly drastic happens: The weekend of camaraderie and occasional misadventure appears to cement certain relationships, dissolve others and activate still others, but the cumulative effect suggests a turning point. Without being completely conscious of the fact, this small circle of friends is transformed by a sense of resignation to adult obligations andimperatives.

The reunion is hosted by Mike (Bruce MacDonald) and Katie (Maggie Renzie), who live together and teach high school history and English, respectively, in Mike's home town. Two other young unmarried couples and two unattached guests are expected. Irene (Jean Passanante) arrives with her new boyfriend, Chip (Gordon Clapp), a straight arrow who is a sitting target for the group's sarcasm; Irene and Chip have met in Washington while working as speechwriters for a liberal senator. Half the second couple arrives: Maura (Karen Trott), an aspiring actress, tearfully reveals that she and Jeff (Mark Arnott), a counselor at a methadone clinic, have busted up.The unattached happen to arrive together, because Frances (Maggie Cousineau-Arndt), a medical student, gives a lift to the hitchhiking J.T. (Adam LeFevre), a struggling count troubadour.

On the first night Maura seeks consolation in the arms of J.T., an impulsive arrangement that stuns the hosts and saddens Frances, who happens to be bedding down only a few feet away on the living room floor and had good reason to believe that she and J.T. had a certain understanding for the weekend. Comes the dawn, and Jeff arrives -- a little too late to make things up with Maura. Trying to be both candid and comforting, J.T. tells the latecomer, "We slept together, but it was mostly just friendly."

The characters are more difficult to sort out in a synopsis than they are on the screen, where their looks, personalities and speech patterns are humorously distinctive. Sayles can create a remarkable number of voices, and his skill doesn't vary from men to women or twosomes to roomfuls. No one but Irene and Chip could share an exchange like the following: "I told you I'd slept with a lot of guys." "You did. I didn't expect to meet any of them."

A different sort of familiarity emerges when Katie and Mike talk, or Maura and Jeff. In addition, the basic group is augmented by two outsiders: Sayles himself as a melancholy but committed young family man named Howie; and David Strathairn, the sharpest comic actor in the cast, as a deceptively happy-go-lucky gas-station attendant named Ron. Former high school classmates of Mike, they have also remained in the old home town, but class and cultural differences, appear to separate them decisively from the collegiate group.

One of the keenest perceptions in the script is that those differences, real as they are, also obscure amusing, affecting similarities. The snobberies of the group that would seem to exclude a Chip, Howie or Ron are ultimately undermined by the fundamental decency and likability of these guys. In the most satisfying reversal of expectations, the wickedly quick-witted Ron succeeds in endearing himself to Frances, who has always regarded him as something of a slob.

Already an accomplished writer, Sayles appears to need only a little more time, polish and financial cushion to evolve into an equally accomplished director. Although conversation is the principal expressive device in "Secaucus Seven," Sayles brings subtle and inventive pictorial intuitions to this directing debut. Comic vignettes depicting a hotly contested half-court basketball game, an afternoon at the old swimming hole (the women sit and gossip and while the men dive nude) and a frenzied bout of woodchopping by one frustrated character illustrate a flair for visual shorthand. The shakiest moments occur in the opening reel, when the characters are being introduced and reunited. It's apparent that Sayles could use a better line reading here and a cleaner cut there. But once the roll call is over, the movie begins to unfold with steadily increasing attractiveness and technical assurance.

The wonderful title derives from a nicknamed coined when some members of the group failed to make it to an anti-war rally in Washington during their college days. The expedition ended in Secaucus, N.J., where police arrested the seven passengers and kept them in a segreated cell overnight. The nostalgic absurdity of it all comes flooding back when the friends drive home from a local bar, stop to investigate a strange object in the highway and get pulled in temporarily for slaughtering a deer, "bambicide," in the terminology of Jeff, who has the longest rap sheet in the crowd.

Still, not even a false arrest is the fun it might have been 10 years ago. As one of the characters confesses, "I used to be able to get stoned, carry on all night and not even feel it." Sayles' small gem of a film pays compassionate comic respects to the contradictions that confront a generation that has begun to feel its age.