QUIET COLUMBIA GROUPS 'TAP' SENIORS

Late the other night, a dozen Columbia University seniors abruptly pulled Brian Margolis, a junior, from his dormitory room into a waiting elevator. They surrounded him threateningly, and when the elevator started to move, Dennis Klainberg grabbed the bewildered Mr. Margolis by a shoulder.

When they left the elevator a minute later, Mr. Margolis had not received a beating or threat, but a ''tap,'' or invitation, to join the Sachems - one of Columbia's two societies for seniors.

The Sachems (prounounced SAY- comes) and the Nacoms (pronounced NAH-comes) are aimed at performing ''discrete service'' to Columbia College. The organizations perform their activities so quietly, however, that many students do not even know they exist.

''I had never even heard of the Sachems,'' said Charles E. Elder Jr., a Columbia College junior involved in the varsity show and gospel choir who was tapped recently. ''Someone had once mentioned senior societies to me, but I didn't know that they actually existed.'' Columbia College is the undergraduate school of Columbia University. Rings of Membership

The Nacoms and Sachems, whose names come respectively from the Mayan and Algonquin words for ''leader,'' each wear a gold pinky ring - one with black diamonds worn on the right hand for Sachems, and one with a green zigzagging line and worn on the left hand for Nacoms.

The Nacoms, the older of the two groups, was founded in 1898. It has an initiation in which members wear robes, carry candles and lead blindfolded initiates through an obstacle course in the campus chapel. The Sachems, founded in 1915, initiate their members with a champagne party.

Members of the groups say that, besides the camaraderie and experience of being in the societies, membership provides connections with former members who are helpful in finding the students jobs and furthering their careers.

''I was able to obtain an interview at a particular bank because a Sachem alumnus works there,'' said a leader of the Sachems, 22-year-old Jaques F. Augustin of Brooklyn. Chosen by Previous Group

Each group consists of 15 ''student leaders'' who are chosen by the members of the previous year's group on the basis of participation in extracurricular activities, character and dedication to the college.

While there are a number of student politicians, athletes and presidents of fraternities, this year's Sachems and Nacoms also include the leaders of the National Society of Black Engineers, the marching band and the movement to divest Columbia's holdings in companies that operate in South Africa.

Each society meets weekly with a dean-adviser to discuss campus issues and to plan projects to benefit the Columbia community. These focus mainly on raising funds. This year the two groups are raising money for a scholarship for needy students. ''The society doesn't really have a purpose - it's just a chance and an honor for the so-called leaders of the class to meet and share ideas and experiences,'' said Richard D. Armstrong, a Nacom who heads Columbia's Coalition for a Free South Africa. He also played varsity basketball. 'Very Positive Experience'

''It's given me a chance to sit down with people from a very different sector of the university, sit down and debate,'' he said. ''That's been a very positive experience.''

''We get involved in certain things on campus that need help to get done,'' said Stuart L. Garcia, a Sachem who is a Columbia College representative to the University Senate.

Columbia College's dean of students, Roger Lehecka, who advises the Sachems, said the group is invaluable to him as a sounding board because it represents a cross-section of the student body. ''We'd have to invent something like it if it didn't exist,'' said Mr. Lehecka, a 1967 graduate of Columbia College who was a Sachem.

Each group also has close ties with its alumni, who endow the societies with ''modest'' financing, according to Mr. Lehecka. Neither he nor James Parker, dean of students at the School of Engineering and adviser to the Nacoms, would comment about the size of the groups' endowments.

Students in both groups said their classmates did not have much of a reaction to their involvement in senior societies. But some other students seemed baffled by the organizations.

''I think it's silly and pretentious, and what do they do for Columbia College or Columbia University anyway?'' said David C. Coplan, an engineering senior from Larchmont, N.Y. ''I think they just rest on their laurels.''