My mother used to be a member of the co-op.

Literal

Mother [topic-は] co-op [in-に] had-entered-[polite-past-ました]

生協 (せいきょう) = consumer cooperative (abbreviation of 生活協同組合). 入っていました = 'had been in / was a member of' — polite past ongoing state. The past tense suggests she's no longer a member.