Maguire has been out of action since suffering an ankle-ligament injury at Aston Villa on 9 May, which forced him to miss our final five matches of the campaign, including the Europa League final penalty shoot-out defeat to Villarreal.
Despite his fitness issues, Harry was selected in Gareth Southgate's 26-man final squad for the tournament, which was confirmed last week.
Although he didn't take part in pre-tournament friendlies with Austria and Romania, Harry joined the rest of the Three Lions' squad - including fellow Reds Dean Henderson, Luke Shaw and Marcus Rashford - during an open session at St George's Park on Thursday morning.