There'll ve a novel "Show Case" offering in the Kennedy Center's Musical Theater Lab at 10:30 a.m. Nov. 30, and the audience will be as novel as the idea.

Only theater party agents and group leaders will be invited to this 90-minute foretaste of what the Kennedy Center will be showing during the winter. A common New York custom, where group sales are important to theaters and party agents, alike, this will be Washington first.

Group Sales director for the Kennedy Center, Chuck Bright, will introduce Martha Scott, whose Plumsted Players are presenting December's "The First Monday in October": Geoffrey Holder, director and designer of "Timbuktu." the musicalized "Kisnet," Gene Barry, star of another new musical, "Spotlight"; stephen Simon, conductor of the January Handel Festival, and the Center's Martin Feinstein, who will discuss its rich ballet season.

The rooftop theater seats unde 200 but already there's a jam on invitations to this once-only sampler1.

With Arena Stage's "The Public Health" and the Kreeger's "Nightclub Cantata" in their fina two weeks, the group's Old Vat is readying "Starting Here, Starting now" to run Nov. 22 through Dec. 18. This non-subscription attraction by Richard Malby Jr., and David Shire, was introduced last spring at the Manhattan Theater Club by three performers. The 165-seat room will be set up cabaret style, with beer, wine and snacks available. After previews, the official opening will be Nov. 26.

The Folger Theater Group will add sign language interpreters for the deaf to a performance of each of its productions this season, drawing on the experienced ones of Gallaudet College. "Teeth 'n' Smiles" will present such an aid Sunday night at 8.

"The Beside Network" of Veterans Hospital is adding to its list of volunteer technicians for its taped radio plays. The recruiter for Jinx Murray, chief of Volunteer Services, it Mary Jo Douds, 466-6263.

Hayloft Dinner Theater, where "Mister Roberts" is a sell-out, has added a revue, "Backstage-Onstage," which is presented performance nights at 11:30 in the theater silo's Starloft Lounge; reservations at 591-8040.

Washington attorney Gary R. Frink is going into show biz to produce "Big Man - The Legend of John Henry," with music by the late Cannonball Adderley and his brother, Nat. This began as a two-record album in 1975 and was a Newport Jazz Festival feature in Carnegie Hall. That's comparable to the route taken by "Jeses Christ, Superstar," Paul Avila Mayer and George W. George have written the libretto for this hopeful folk musical. Frink aims for a Washington tryout if a theater can be found.

"The Merchant" regained director John Dexter after his staging of the Metropolitan Opera's "Rigoletto" and has shifted its New York stage to the far smaller, more intimate Plymouth, opening on Nov. 16. In its final performance at the Eisenhower, John Wesker's play had been much improved with a re-staging of the trial scene and with 20 minutes less of the script. Gotham previews began last night.

"Off the Wall" is the title of Street 70's Mime Troupe production opening Friday in Rockville Civic Center. The cast includes director Will Robertson, Jerry Whiddon, Michael Littman, Gayle Behrman J. Ben HOoks, Richard Averbach and Lisa Mandle. Reservations at 463-4172 for Thursday through Sunday performances this weekend and next.