The Reds have signed the 22-year old Sydney-born striker on an initial four-week deal following a seven-year stint with Danish club Brondby.
"Brent has impressed in the two pre-season games he's played and he has over 40 games of first team experience with one of the best clubs in Denmark," United's football operations manager Michael Petrillo said.
"He's only 22 years of age and can play as a centre forward or out wide so we think there is a lot of upside to him."
McGrath added: "It will be a good experience. They are a really good group of guys here, there's a good coach, I feel like I fit in with the style of football the team plays and it's a good chance to prove myself and hopefully extend my stay for longer."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball