Generally, it is not illegal to associate with other people.  However, street gangs pose a serious problem because they are formed with the purpose of committing crimes.  For that reason, being a gang member can give rise to conspiracy charges. Gang involvement can also add 10 years to a prison sentence for committing a violent crime or a crime involving a controlled substance.

S.C. Code Section 16-1-410 and 18 U.S.C. 521.