As a measure word, 一伙人 means 'a group or gang of people', sometimes with a negative connotation (like 'a gang'). 大伙儿 is a friendly colloquial way to say 'everyone'.
Left person radical — the indexing component. 伙 is fundamentally about people gathered together: companions, partners, a band. The radical puts 伙 alongside other group/relationship words: 伴 (companion), 们 (plural marker), 你 (you), 他 (he).
Right fire — supplies the sound directly (huǒ = huǒ, exact match) and a vivid semantic image: a group of people gathered around a campfire, sharing warmth and food. That hearth-sharing scene is the original sense of partner/companion that 伙 carries.
