Top is 取 'to take, fetch' (an ear 耳 plus a hand 又) - the original ear-grabbing graph for seizing war trophies. It supplies the sound (qǔ → jù, q/j alternation) and adds semantic flavor: 聚 is gathering by actively taking. Filed under Kangxi #128 耳 (the ear radical embedded inside 取).
semantic
乑yín
many people standing together
Bottom is 乑, a rare component picturing three figures (人人人) packed side by side - literally a crowd. It is the same building-block underneath 众 crowd. So 聚 reads visually as 'take + crowd' = bring many bodies together. Hence 聚会 to assemble, 聚集 to converge, 团聚 family reunion.
Filed under radical 耳 (ěr, #128) by convention. 耳 is not a separate component in 聚, so no strokes are highlighted.
