According to dictionary.com, “committee” comes from late Middle English, with the suffix -ee added to the word “commit”. Typical use of the -ee suffix would imply the meaning of “one who commits” or “one to whom something is committed”. Wiktionary agrees, and I understand that both of these meanings see usage in British Parliament today, according to Oxford.
When and why did it become a collective noun meaning “a person or group of persons elected or appointed to perform some service or function” instead of only a single person?
With emphasis on the final syllable, ‘committee’ is recorded earlier, from 1472-3, than ‘committee’ with emphasis on the next to the last syllable, from 1566. The ‘-ee’ suffix of the earlier word has the more usual sense:
…freely added to English vb.-stems to form nouns, … those in -ee [denoting] the passive party, in such transactions as are the object of legislative provision.
OED derives the later word from a variant form of the earlier. That variant is shown in the earliest quote recorded, that is,
1566 R. Horne Answeare M. J. Fekenham f. 32v The Iudges seeinge the exclamations and confusion..appointeth a Committy, choosinge foorth of sundry partes a certaine number to goe aside with the Iudges, to make a resolution.
OED remarks that the distinguishing stress pattern of the two words
…is suggested by forms like committy (compare -y suffix5) which are attested from the mid 16th cent. (compare quot. 1566 at sense 1a); its origin is uncertain, and may be due to association of the word in senses relating to a group or body of people with classical Latin comitium….
The ‘-y’ suffix, in the sense “suggested by forms like committy“, denotes an
office or function, or the persons performing it….