supernumerary
Supernumerary (noun) means someone who works alongside a group or office but is not one of its regular members.
Etymology
Comes from Late Latin supernumerarius (“extra soldier added to a legion; supernumerary”), from super (“above; beyond”), numerus (“number; collection, quantity”), and -ārius (a suffix forming adjectives from nouns or numerals). The word can also be analysed as super- + numeral + -ary.