More than 38,000 federal and postal workers have signed up for the new tax-deferred thrift savings plan in the past month and the value of their three-month-old pooled investment account is fast approaching the half- billion-dollar mark.

As of yesterday, the fund, which began accepting employe contributions in April, had $440 million. The seed money for the fund came from the government, which since the start of 1984 has automatically opened an account for virtually all new employes and which, each payday, makes a contribution equal to 1 percent of each employe's salary to that person's account.

Currently the fund, which will branch out into the stock market next year, is paying 8 1/2 percent interest. The investment account is now growing by $5 million every day because of stepped-up employe and government contributions and because of earnings from interest. Since mid-June the fund has risen $91 million.

Spot checks with agency personnel officials indicate that federal workers in the Washington area, whose $32,000 salary average is higher than that of their colleagues in the field, are participating at a greater rate and investing a bigger share of their income than are civil servants beyond the Beltway.

The thrift plan is part of the new Federal Employees Retirement System. Most workers hired since the end of 1983 are automatically enrolled in the thrift plan, with the government contributing 1 percent of their salaries into their accounts each payday. Of the 581,495 workers under the FERS plan, 177,318 (or just over 30 percent) are putting their own money into their accounts and getting additional matching contributions from their agencies. Those who contribute 5 percent or more themselves get a 5 percent tax-deferred matching contribution from Uncle Sam. This year workers under the FERS program can put up to 10 percent of salary (or $7,000) in the thrift plan.

Federal workers covered by the old Civil Service Retirement System (nearly everyone hired before 1984) can also participate in the thrift plan, but are limited to making 5 percent contributions. They get no matching government contributions unless they convert to the FERS program during the open enrollment season that ends Dec. 31.

Of the estimated 2 million U.S. workers still under the older pension plan, 321,020 have signed up to have money taken from their paychecks for the thrift investment plan.

Employe participation is critical because it will be used later this year to determine what percent of pay higher-income workers (those making $50,000 or more) can put into the thrift plan. Under so-called antidiscrimination rules covering such tax-deferred contributions, upper-income workers are limited to 2 percent of the average amount contributed by all eligible lower-income workers.

For purposes of fixing the new contribution rate the government will consider the average contribution of all employes in the FERS plan, regardless of whether they contribute, as one group, and the participation rate for all employes under the CSRS retirement system as a separate group.Job Mart

The Government Printing Office has two openings for printing specialists, Grade 12. Call Karen M. Urchasko in GPO's personnel office.

Treasury needs several budget analysts, Grade 12. Call 447-9798.

Internal Revenue Service needs clerk-typists, GS 3 through 5. Call Kristine A. Roth at 376-0662.