How to join a group using SignalR

-------------------------In Javascript (ReactJs)---------------------------------

const connection = new signalR.HubConnectionBuilder()
  .withUrl("connectionUrl")
  .build();
connection.start().then(res => {
    connection.invoke("JoinGroup", "groupName")  //JoinGroup is C# method name
        .catch(err => {
            console.log(err);
        });
}).catch(err => {
            console.log(err);
        });;

----------------In C# (.Net Core)-----------------

public class NotificationHub : Hub
    {
        public Task JoinGroup(string groupName)
        {
                return Groups.AddToGroupAsync(Context.ConnectionId, groupName);
        }
    } 

You can't. If you could join a group from javascript then anyone may use your code to join any group which breaks security. If you really need to do that - create a method on the server side that takes a group name as parameter and adds the client to the group.

public void JoinGroup(string groupName)
{
    this.Groups.Add(this.Context.ConnectionId, groupName);
}

Afterwards, call it from JS like that

eventHub.server.joinGroup("my-awsm-group");

Just in case you come across this question now (like I did), here is an example for how to implement an azure function to support groups.

https://docs.microsoft.com/en-us/azure/azure-functions/functions-bindings-signalr-service#2x-c-group-management-output-examples