James didn't do a particularly good job during the preseason, but he apparently did enough to earn himself a spot on the roster. In the preseason he put up 95 rushing yards on 16 carries, averaging 5 yards a carry, and he also caught 3 passes for 8 yards.
As a returner however, James had a lot of struggles. He muffed a punt against the Tampa Bay Buccaneers, and he slipped and fell during his first punt return against the Chicago Bears. Kick returns were not much better.
Nevertheless, James' training camp performances and shiftiness on the field as a running back were enough to earn him a spot on the roster, and with reports out that Jay Ajayi will likely make the roster as well, the Dolphins seem to be set to go with four running backs, as they did back in 2014.