The Spanish verb 'agregar' (meaning 'to add' or 'to join') comes from the Latin word 'aggregare', which meant 'to add to' or 'to accumulate'. The Latin word itself was formed by combining two parts: the prefix 'ad-' meaning 'toward', and 'gregare' meaning 'to gather'. When 'ad-' was joined with words beginning with 'g-', the 'd' typically changed to 'g', giving us 'ag-' + 'gregare'. This combination perfectly captures the sense of bringing things together or adding to a group.

A related Spanish word is 'congregar' (to congregate), which shares the same Latin root 'gregare'. While 'agregar' emphasizes adding or joining something to an existing group, 'congregar' focuses more on gathering people or things together in one place. Another related word is 'segregar' (to segregate), which uses the prefix 'se-' (meaning 'apart') instead of 'a-', thus meaning to separate from the group rather than join it.

English speakers might recognize this etymology in words like 'aggregate' (a group or sum of things gathered together), 'congregate' (to gather together), and 'gregarious' (fond of company, sociable). All these words share the Latin root 'gregare' meaning 'to gather'. The English word 'aggregate' is particularly close to the Spanish 'agregar', as they both derive directly from Latin 'aggregare' and maintain similar meanings of bringing things together or adding to a group.