Reacting to a rapid rise in the number of shares of stock held for investors by third parties, a blue ribbon advisory committee yesterday recommended a series of measures to improve communications between corporations and millions of stock owners whose shares are held by others.

The Advisory Committee on Shareholder Communications, appointed by the Securities and Exchange Commission, said that many of the companies it had contacted reported that a majority of their stock was held by "nominees."

Although in many cases customers know that their stock is held by a nominee, in some cases they don't. A customer may choose to have stock held by a nominee because he or she doesn't want the investment disclosed, it may be held by a nominee because brokerage rules require it, as in the case of margin accounts, or it may be done that way simply to make transactions more convenient.

"I think a lot of customers are not aware that they are doing it and what that means," said committee Chairman Paul D. Weiser, senior vice president of Dataproducts Corp. The committee recommended that the stock exchanges and the National Association of Securities Dealers prepare a brochure explaining to investors the ramifications of having stock held by a nominee.

The nominee system was set up to improve efficiency in processing stock transactions by eliminating the need to physically transfer stock certificates. Under SEC rules, a corporation must supply brokers, banks and other depositories with sufficient proxy materials and other information to distribute to the actual, or "beneficial," owners of the stock.

Under the system, an individual who buys shares in a company may have the stock registered in the name of a depository, such as the Depository Trust Company. The individual receives annual reports and proxy material through the nominee but does not hold the actual stock certificates. For example, when investor A sells his or her shares to investor B, no physical transfer of stock need occur--just a bookkeeping transaction.

As one example of how the process has grown, the committee noted that the number of equity shares held by the Depository Trust Co., which is one of the three major depositories, had increased from 4 billion in 1976 to 19 billion by the end of 1981.

Although the system has helped with paperwork, sometimes it has been a barrier to communications between corporations and shareholders, the committee found. The committee said that it found, with respect to proxy voting, that "at least some companies have reported a slow, steady decline in voting on major questions over the past few years and express serious concern about the future."

Banks, which are not regulated in this area by the SEC, now hold more than one-half of all stock registered in the name of nominees, Weiser said. The committee noted that banks are making efforts to improve communications with beneficial shareholders, but also recommended legislation authorizing the SEC to adopt rules applicable to bank nominees and placing the responsibility for enforcing those requirements with federal bank regulators.

The committee also recommended that a means be provided to allow corporations to identify beneficial owners who consent to disclosure.

Lee Spencer, director of the SEC's division of corporate finance, said that the SEC probably will consider the recommendations that would require agency action within the next few months.