And it all started with a successful tryout.
Guinn was put on the practice squad two days before the 2015-16 season began. Then he went so hard in practice he annoyed some of the team's stars. But it was worth it, he said.
When Cory Calvert decided to transfer and Jake Toolson took a leave of absence — both players are now at Utah Valley — Guinn was added to the roster.
"That was a great moment for me," he said.
BYU coach Dave Rose said they were aware of Guinn because the Cougars' former director of basketball operations, Cody Fueger, had helped recruit Guinn to UC Riverside when he was on that staff
"In the walk-on tryouts, what really impressed us was his diversity, his ability to do so many things in the short period of time that we saw him," Rose said. "He shot the ball well, played hard. He was a pretty good defender on the perimeter. He was just a little bit advanced from that group, and that obviously had to do with the experience he had when he played a year of Division I basketball."
Guinn averaged 2.9 minutes, 1.6 points and 1.1 rebounds per game his first season at BYU and was so strained financially without a scholarship he figured one season as a Cougar might have to be enough. But again, hard work, perseverance and determination paid off.
More scholarships opened up last summer when Jakob Hartsock decided to focus on academics and Jordan Chatman transferred to Boston College, and Guinn was awarded the financial aid.
"The pieces all fell into place for me," he said. "As far as the opportunities that came, I just feel blessed that they happened, and all that, combined with hard work and some faith from coach Rose and the other coaches just kind of allowed me to have a chance."
When Rose announced to the team in the weight room that Guinn was being put on scholarship, "all the guys were cheering and yelling," he said. "It was an awesome moment. But these are special guys here. At some schools, if you are a walk-on, they treat you maybe a little bit differently. These guys treated me just like I was a scholarship guy all along."
Guinn made a splash early in the season when he came off the bench to go 3-for-3 from 3-point range and score a career-high 11 points against Valparaiso, but he's really asserted himself the past four games as Rose has employed a 1-3-1 zone defense with the 6-foot-5 wing at the top pressuring the ball-handler.
He's averaged 15 minutes per game the past four contests, and should be a big part of the defensive game plan on Thursday against Pepperdine and talented senior guard Lamond Murray Jr., the WCC's leading scorer (20.7 ppg.).
Rose said Guinn has gone from tryout invitee to main rotation guy because of his relentless effort.
"He is deceivingly long, too, and he plays with just a tremendous amount of energy, and he is just determined," Rose said. "We just told him we want him to get his hands on the ball as many times as he possibly can from that position, and he has taken that challenge, and was really good both these games this past weekend."