Getting ConnectionId for known UserId, or adding User to Group

Question

I'm writing an application which uses Azure SignalR in serverless mode, since I need to transmit messages from Azure Functions.

To that end, I'm using the Microsoft.Azure.SignalR.Management library to access a hub context so I can send messages. That part is working so far, both when the call is made from Azure Functions, or from a RESTful api call.

I would like to be able to add a specific user to a group. The user I can identify using a userId

 public async Task<ActionResult> AddToGroup([FromBody]GroupAddRequest request)
    {
     
        //This crashes - since it expects a connection id instead of a user Id
        await this.messageHubContext.Groups.AddToGroupAsync(request.UserId, request.GroupId);
        await this.messageHubContext.Clients.User(request.UserId).SendAsync("group", request.GroupId);
        //I can send a message to a specific user ^

        return new JsonResult("Done");
    }

I can send messages a user identified from the userId - but I can't add that user to a group since it's expecting a connectionId instead. So how can I get the connection of that specific user, or alternatively - how can I add a User to a Group. I can't use "Context" since this isn't within a hub.