The club members decided to give each senior a brooch, one at a time.
Literal
Club-members [topic-は] seniors [to-に] one-person-at-a-time brooch [object-を] give [decided to-ことにした].
Uses ~ことにする (decide to do) and ~ずつ (each/at a time). 先輩 (senior/upperclassman) is a key social relationship term in Japanese school and workplace culture. ブローチ is a loanword (brooch).