How to map single MC Connect API User with multiple SF System Users?

Definition Account-Level / user-Level Connection In your current setup, you have completed Account-Level Connection via the API User / System User pair, so you see most things working just fine. You have not established User-Level Connection for the API user / System User pair. If you have no problem, I would say there is no need to do anything. Nothing (except maybe missing licenses) stops you from establishing User-level connections for other users in your system when necessary. Effects of not integrating users Everything that requires a user-level connection (i.e. a context of "who is executing something") is not possible without mapping users. This would include also scope by user, which is not your usecase. other examples: e.g. unmapped users will not have access to the "Salesforce Email Send" activity in Automation studio. If you login as your MC API user, you should not see this activity type in Automation Studio. Bit of a rare case but that's one visible difference. This likely also affects Triggered Sends from Sales Cloud, but I haven't tested this. With Journey Builder being what it is, this has become pretty rare in my experience anyways. Connecting several users in one org to the same MC API User As for your intended configuration - not possible as per the documentation: "You can’t map a single Marketing Cloud user to more than one Sales or Service Cloud user within one org." ...and also phrases it the other way around: "You can map a single MC connect API user to multiple Sales or Service Cloud users if the Sales or Service Cloud users are in different orgs and the Marketing Cloud user has access to the connected business units."