Former Lib Dem leader Charles Kennedy, ex-chief secretary to the treasury Danny Alexander and former Scottish secretary Michael Moore were voted out of Westminster amid an SNP landslide at the general election.
When asked if they could be persuaded to stand for Holyrood on STV Scotland Tonight, Mr Rennie said he would “always encourage them to do so”.
“I think it’s too early to tell,” he said.
“They need to take time to come to terms with the result, but I have been in regular contact with them and I am encouraging them to play an active role because they are great Liberals.
“They have contributed towards the well-being of the country for some time and I would like them to continue to do so.”
He added: “I think there is no doubt that people of the standard of Charles Kennedy would fit in well in the Scottish Parliament.
“They would join this very strong Liberal Democrat group that we have already got at the Parliament and be a big voice for Liberal values. So I would always encourage them if they would like to do so.”
When pressed on whether Mr Kennedy would be a candidate for 2016, Mr Rennie said: “It’s up to him to decide.
“He is obviously taking time, along with the others, to decide what their future holds and I would never rush them in making that decision.
“These are quality people who have contributed to the well-being of Scottish society for some time and it would be fantastic to see them continue to play a big role in politics.”