。
Please drop by the office in the evening.
Literal
Evening company [at-に] [humble-お]-drop-by-[polite-ください].
お立ち寄りください is the honorific request form: お + verb-stem + ください elevates the listener's action. 立ち寄る ('to stop by, drop in on the way') is a useful compound verb — 立つ (stand) + 寄る (approach) — for brief visits. The whole construction is a standard polite invitation used in business or hospitality contexts.