The Activity Index and Activity Show API Endpoints do not currently include one of the most important attributes of an activity, who completed it. According to support, the only way to get this information is to implement webhooks and track them as they are completed. This does not help for historical activities.

I highly recommend adding CompletedByUserId to the Activity Index and Activity Show API Endpoints. It is invaluable data for your clients. The activity reports native to Redtail already have this field available.