The Group Health Association and its striking doctors union agreed to resume negotiations today to try to end the two-day strike, which expanded yesterday with many GHA nurses staying off the job in sympathy with the physicians.

The 145,000-member health maintenance organization said it had maintained basic service yesterday at its seven Washington area centers, although GHA officials said that a substantial number of the 200 nurses and physical therapists had not reported to work, along with the majority of the 160 unionized doctors.

The association beefed up its staffing yesterday by hiring temporary nurses and by referring hundreds of GHA patients to private doctors. Management physicians and several nonunion doctors worked double shifts.

Each side in the dispute, while agreeing to resume bargaining at the Federal Mediation and Conciliation Service headquarters, also filed a legal complaint against the other with the National Labor Relations Board. But both sides also said they hope the strike will be resolved before the NLRB hears the complaints.

Group Health filed a complaint with the NLRB asserting that the doctors' strike is illegal because the union did not properly file a 10-day notice of intent to strike, which is required by federal law for health-care labor disputes. GHA said the complaint was based on a "technical" defect in the strike notice that the union filed two weeks ago, but a union spokesman called the allegation "baloney."

The Capital Alliance of Physicians, which struck Monday in a dispute over working hours and scheduling, filed an NLRB complaint that the association refused to bargain in good faith during weeks of negotiations, seeking a board order to require GHA to bargain. GHA said the complaint was baseless.

The Registered Nurses and Physical Therapists Association also filed an NLRB complaint based on the union contract, which gives nurses the right to stay away from work in the event of a strike by other employes. A union spokesman said a letter sent by GHA last week to employes was an illegal threat of discipline if nurses honored the doctors' picket lines, an allegation GHA denied.