A federal grand jury in Tampa, Fla., is investigating an independent campaign fund-raising group, called the 1980 Republican Presidential Campaign Committee, which operated out of a mail drop on K Street and raised almost $400,000 from Ronald Reagan supporters this year.

A focus of the investigation, according to sources, is American Campaign Consultants Inc., a company that received $278,000 of the money and listed its address in Federal Election Commission records as another mail drop, just a few doors away from FEC headquarters, on K Street.

The grand jury, according to sources, is looking into donors' complaints to the committee that its literature misled them into believing that it was the official Reagan committee. The grand jury is also studying whether the money raised was actually used to support Reagan, and the possibility that American Campaign Consultants and the principals behind the fund-raising committee are the same group.

Among those subpoenaed to appear before the grand jury, according to the Tampa Tribune, are Sheldon L. Wind, a Tampa lawyer, and Ronald A. Gyenis, a Tampa resident who specializes in political fund-raising. Gyenis owns a mailing list of about 35,000 conservative contributors.

Gyenis was questioned during a 1978 FEC investigation into the origins of his campaign funds list. His lawyer at that deposition was Wind. The matter was subsequently dropped.

Wind serves as treasurer of the 1980 pro-Reagan committee, but has refused, in past interviews, to identify the principals of his committee. They simply "don't want to be known," Wind said last month. He has maintained, however, that he did "nothing wrong."

In the committee's latest report, filed last week, Wind disclosed that another $50,000 was paid to American Campaign Consultants Nov. 3, the day before the election, for "consulting."

On Nov. 12, a day after a story on Wind's committee appeared in The Washington Post, Wind set aside another $21,000 "for current and future legal fees for the committee."

The grand jury is expected to resume taking testimony the first week in January.