Although only a few thousand federal workers have voluntarily moved into the government's new retirement plan, called FERS, nearly a million of Uncle Sam's 2.8 million civil servants have signed up for a private- enterprise option in the new pension program that lets them become players in a government-guaranteed, tax-deferred investment plan.

By Dec. 31 each of the 2 million federal workers hired before 1984 must decide whether to remain in the old Civil Service Retirement System or move into the new Federal Employees Retirement System. FERS automatically covers nearly all workers hired since the end of 1983.

The FERS plan offers one investment option for employes under the old pension plan and a more generous option for those in the new system. Employes under the CSRS can invest 5 percent of their pay into the tax-deferred thrift plan that is paying 9 percent interest.

Workers fully covered by the FERS program can invest 10 percent of their salary (up to $7,000 this year) and anyone putting in 5 percent or more gets a matching 5 percent tax-deferred contribution from the government. Even FERS workers who do not contribute anything have accounts opened for them by the government, and get a 1 percent of salary contribution each month.

Money invested by workers through payroll deduction is not considered as taxable income (nor are earnings from those investments) until the money is withdrawn. The government contribution to those accounts amounts to a tax-deferred pay raise.

The thrift investment plan started in April and already is worth more than $705 million, and is growing at the rate of $5 million a day.

As of last Friday, 219,374 employes under the FERS system were contributing their own money into the thrift plan, and 392,898 other workers under the FERS system had accounts that contained only the 1 percent government contribution.

Of the nearly 2 million workers still under the old CSRS pension plan, 372,368 were voluntarily putting in amounts ranging from 1 to 5 percent of salary.

All money invested in the thrift plan this year goes into a so-called G-fund made up of securities issued by the Treasury.

Next year, workers who are under the FERS program can elect to put a part of their investment into any (or all) of three funds, including the G-Fund, a C-Fund made up of common stocks or an F-Fund composed of fixed income securities and bonds. Workers who elect to remain in the old pension plan can invest only in the G-Fund.

The next open season for the thrift fund begins Nov. 15 and runs through Jan. 31. During that time employes can change their investment levels, withdraw or join the plan.HMO Moves

Health Maintenance Organizations that provide prepaid medical services are expecting a big increase in membership next year as many federal workers and retirees move out of more traditional health plans whose premiums are going up dramatically in 1988.

Many of the HMOs are posting only modest premium increases and some are holding the premium line. Workers and retirees have an open enrollment period, from Nov. 9 to Dec. 11, when they can shop around for new health insurance plans.