The details of the contract were not released so it's entirely possible, likely even, that this signing is non-guaranteed and is just a training camp contract. He'll get his shot to impress the team during training camp and stay on after the roster is trimmed to a maximum of 15 before the beginning of the season.
Career details can be found in the Official Release:
SPURS SIGN JAMYCHAL GREEN
SAN ANTONIO (August 6, 2014) - The San Antonio Spurs today announced that they have signed forward JaMychal Green. Per club policy, terms of the contract were not announced.
Green, 6-8/230, was a member of the Spurs 2014 Summer League team in Las Vegas. In six games he averaged 7.8 points and 6.5 rebounds in 16.2 minutes.
The Montgomery, Ala. native spent last season playing in the French Pro A League for Roanne, averaging 11.8 points and 6.6 rebounds in 22.1 minutes in 25 games while shooting .530 (122-230) from the field.
Green spent the 2012-13 season with the Austin Toros of the NBA Development League, averaging 12.3 points, 8.1 rebounds and 1.4 assists in 40 games.
An undrafted forward out of Alabama, Green averaged 14.0 points, 7.4 rebounds and 1.5 blocks his final season for the Crimson Tide en route to being named All-SEC First Team in 2012.