会餐 is a separable verb-object compound (离合词) meaning 'to dine together' as a group activity. The verb 会 and the object 餐 can split when adding measure words, durations, or other modifiers between them — for example, 会了一次餐 ('had a group meal once'). You cannot place a direct object after the whole word; instead, use a structure like 跟同事会餐 ('dine together with colleagues').