Gregarious \gri-GAIR-eei-uhs\ , adjective;
1. Fond of the company of others; sociable
2. Living in flocks or herds, as animals
3. Botany: Growing in open clusters or colonies; not matted together
4. Pertaining to a flock or crowd
This word dates to the 1660's from Latin gregarius ("pertaining to a flock; of the herd of the common sort, common"). Gregarius comes from grex ("flock, herd"), which is a reduplication of the Proto-Indo-European base *ger- ("to gather together, assemble"). The sociable sense is first attested in 1789.
Segregate, aggregate, egregious, congregate, and category are all relatives of gregarious through either grex or *ger-.