Scottish internationals McStay and John Collins, as well as the long-serving Republic of Ireland goalkeeper, Pat Bonner, and midfield player Peter Grant are taking advantage of the club's current share issue. All four are to invest in Celtic before the 10am deadline on 24 January.
Celtic hope to raise £9.4m through the share issue and say they have received over 7,000 enquiries for details and more than 25,000 fans have been mailed information. Dermot Desmond, an Irish investor, has already bought £4m worth through his company Quay Financial Software Limited.
McStay, who wants to stay at the club when his contract expires this summer, said: "I'll definitely be buying shares. The chance is there for everyone who can afford it to have their say in what goes on at Celtic."
Collins, the top goalscorer so far this season, said: "It's a good thing to do and something I can hand on to my children.
"It's nice to know you have got a small part of the club which, I'm sure, is the way everyone will feel."
Meanwhile, Celtic will today put on sale tickets for their Scottish Cup third-round tie with St Mirren at Hampden on 28 January.
Gillingham fans are to dip into their own pockets to help the manager, Mike Flanagan, in his attempt to keep the Southend defender Mick Bodley on loan at the Kent club for a third month.
The Supporters Club chairman, Alan Liptrot, and the secretary Peter Lloyd have agreed to make a £2,500 donation to the club. Part of it will help to pay Bodley's wages and £1,000 will go towards restocking the club's souvenir shop.
Liptrot said: "We had a meeting to discuss the club's position and wanted to help in any way we could, so we decided to make a donation to support the club in these difficult times."
The debt-ridden Third Division club, who are being run by a receiver, have been put up for sale and around 1,000 fans are to turn out for a public meeting at the Priestfield Stadium tonight to hear the latest developments in the club's fight for survival.
They will be addressed by the receiver, Tom Burton, and the major shareholder, Tony Smith.