Monfries, ruckman Matthew Lobbe and defender Tom Jonas have joined the group for the looming season.
The trio join a leadership group headed by captain Travis Boak and vice-captain Brad Ebert and also including Robbie Gray, Jackson Trengove and Hamish Hartlett.
Monfries joined Port last season and, with Lobbe and Jonas, showed great qualities, Power coach Ken Hinkley said.
"What they've been able to do, not purely just on the football ground but around the club and at training as well, is demonstrate great, young leadership," Hinkley said in a statement on Monday.
"They are people who want to take the club in the right direction."
Veteran forward Jay Schulz was the only omission from last season's leadership group.
"There comes a point in a player's career where he acts as a leader without needing to be in a leadership group," Hinkley said of Schulz.