The group was originally led by Lazard Deusericus in Crisis Core, and is divided into three classes: 1sts, 2nds, and 3rds, with 1sts being the best. The classes are separated by their uniform colors. 1sts wear black uniforms, 2nds wear purple, and 3rds wear blue. 1st Class SOLDIERS, it would seem, have permission to add to and subtract from the standard uniform.
SOLDIER members have included Zack Fair, Angeal Hewley, Sephiroth, Genesis Rhapsados, and Kunsel. Nero and Weiss may have also been members of SOLDIER, having been seen wearing SOLDIER 1st class uniforms in Crisis Core. Cloud Strife also claims to have been in SOLDIER, but this is later disproved after realizing he had taken on Zack's memories.
Due to colorization, Cloud's 1st class uniform in Final Fantasy VII often looks purple, but when shown in the FMVs, it is appropriately black.
Zack in a 2nd Class uniform from Crisis Core
Angeal in a 1st Class uniform from Crisis Core