Boardman played alongside MacDonald at Woking in the Conference (National League) and have had kept in touch since.
Boardman said: "Having Gary on board is a great boost for the team and will be another footballing brain on the sideline to help myself and Spud. We welcome Macca to Bulpit Lane and look forward to using his experience."
MacDonald began his career with Portsmouth but made no first-team appearances and joined Havant & Waterlooville.
He joined League One club Peterborough United in February 2001 but struggled to break into the first-team, making twenty-one appearances in two years.
He joined then-Conference side Stevenage Borough on loan in November 2001 and made eleven appearances during the remainder of the 2001–02 season.
At the end of the season, he joined Woking where he started 120 games in four years and played alongside Boardman.
He was signed by Ebbsfleet United in June 2007 but suffered a ruptured Achilles tendon in his first game in August 2007, which ruled him out of the game for several months until he made his comeback in a Kent Senior Cup match in January 2008.
MacDonald returned to the first-team in March 2008 and by the end of the 2007/08 season, had made eight appearances for Ebbsfleet and collected a winners` medal when they won the FA Trophy at Wembley in May 2008.
MacDonald returned to Havant in 2009, but fell out of favour with the manager and made his last appearance for the club in March 2010.
During the close-season of 2010, MacDonald signed for Kingstonian.
Following a one-month loan with Metropolitan Police, the 34-year-old signed for Walton Casuals. In July 2014, the versatile defender signed for Slough Town. He then briefly lined up for Farnborough, before a switch to Staines Town.
He then joined Walton & Hersham in February 2016 and following the resignation of manager Mark Hams the following month, MacDonald was announced as player-manager for the eight remaining games of the 2015/16 season.
In July 2016, he was announced by Hayes & Yeading United to be Mickey Lewis' playing assistant-manager, having had two previous spells with the club, most recently in 2015 under Phil Babb and Tristan Lewis.
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