How to show online members on our website ?

I think you can use following approach for this

Just store sessions (If you really use session) in separate tables in your database and then manipulate them dynamically (This is not a best approach , but quite easy you can use it)

I don't think database is used there because if we use database then for every message show in group chat i have to establish a connection between database and controller.And that degrades the performance of group chat.
Same thing can be utilized to show the online members.
I want to store that information's in List.
On every session creation i want to update the list and on refreshment of page it show's the every elements of lists by iterating them.

I used application scope to store the information and loads in every session but still it's not working for me.
I think one more problem is there our server load increase due to heavy memory use isn't it ?