Earlier, Nathaniel Fearnley anchored the PB batting effort with an excellent 46 off 52 balls, including five fours, at the top of the order.
McGuinness (21 off 17, with four fours) and Reynolds (18 off 22) contributed vital runs.
Jai Poonia made 43 off 44 for BoP before being the last wicket to fall on the last ball.
It was Poverty Bay’s second win of the tournament, having beaten Northern Māori by one wicket in a 50-overs match on Day 3.
Northern Māori had a great final day, winning both their matches to jump ahead of PB into sixth place on the ladder on run rate.
Waikato Valley were the stand-out team of the week, winning all seven matches.
The ND age group representative action continued with the Rangatahi Youth Tāne U17 tournament in Hamilton this week.
The Poverty Bay team competing are Charlie Castles, Cody McMurray, Joel Kirkpatrick, Patrick McInnes, Robbie Newlands, Harvey Reynolds, Finnbar Whitfield, Gayesha Mahabalage, Raffaele Colucci, Brandon Fearnley, Jack Williams and Pranash Senthooran. Martin Worndl is coach and Graeme Newlands is manager.
Kirkpatrick, McInnes and Fearnley played for PB in the ND U19s tournament, as did Johnny Gray, who is in the Northern Māori team at the U17s.
All four were also involved in the Poverty Bay Premier Grade Doleman Cup club cricket final on Saturday.