An evangelical minister and former aide to President-elect Ronald Reagan who mailed tens of thousands of official-looking invitations to an extravagant two-day inaugural celebration has agreed to tell ticket buyers that his gathering is not the official event and that neither Reagan nor Vice President-elect George Bush will attend.

The agreement, announced yesterday by lawyers for the officially sanctioned Presidential Inaugural Committee, was signed by James E. Johnson, 53, a former assistant secretary of the Navy who organized the Presidential Inaugural Foundation, which has mailed more than 25,000 gold-embossed invitations to a "Presidential Inaugural Celebration . . . With Love" to be held Jan. 19 and 20 at the D.C. Armory.

The agreement, reached earlier this week after more than eight hours of meetings between Johnson, former governor Reagan's secretary of veterans affairs in California; his attorney and lawyers for the official inaugural committee does not affect a continuing investigation of Johnson's foundation by a federal grand jury here, which was begun three weeks ago after the White House and postal officials complained about the invitations.

"This settlement protects the interests of the committee and we hope nobody receiving the invitations will be misled," said Roger Clark, an attorney for the official committee. Earlier this month, Clark informed Johnson that the committee was considering seeking a court order to block the issuance of further invitations, which promoted a "planned visit by the newly elected president and vice president" at the unofficial festivities.

Terms of the settlement include a pledge by Johnson to change the name of the Presidential Inaugural Foundation to the Foundation for Religious Celebration of the Presidential Inaugural, a promise to refund upon request any of the more than $30,000 collected so far at $135 per ticket. He also has to attach a lengthy disclaimer to the more than 40,000 invitations Johnson has already sent or plans to mail. That disclaimer says that neither Reagan nor Bush "has stated that he will be able to accept our invitation to attend."

Johnson could not be reached for comment yesterday but his attorney, John Snyder, denied that his client had defrauded anyone and said Johnson "humbled himself" by signing the agreement "for the sole reason of not wishing to embarrass his good friend Ronald Reagan."

Snyder described the response to Johnson's unofficial inauguration as "overwhelming" although he said he did not know how many tickets had been sold, but it appears to be no more than about 220.

The Armory, which has a capacity of 8,000 people, "will be filled, partly by senior citizens or handicapped people who can't afford to pay" and will be given free tickets, Snyder said. "That's always been Dr. Johnson's plan."

When asked why the invitations made no mention of this and included a "requested donation" of $135 per person, Snyder chuckled and replied, "Good question."