I have a solution contains multiple class library and two web(ui) projects (Web and Admin).
My Hubs are in a class library that two ui projects access to this.
How can i display online users from WEB project in َADMIN project
If you have two separate projects that are in different AppDomains, the Hub must exist in one of the two projects. For example, if you have the Web Project loaded as https://web.azurewebsites.net and the admin as https://admin.azurewebsites.net, you will have two separate Hubs if you instantiate the Hub in each. This is NOT what you want to do if I understand your requirements.
Instead, you should load your Hub in one of the two, which I assume will be the web project. The admin project can set itself up as a SignalR client. Your Hub in the web project can then send updated user counts to the Admin project based on the SignalR onConnect and onDisconnect methods, just as Luke suggested.
For that matter, if the admin project doesn't need any other functionality than getting user counts from SignalR, you could have your Hub in the Web project, and have the Web project call WebAPI methods in the admin project to update user counts.
I made a lot of assumptions about your solution. Please let me know if I'm wrong, and I'll revise or delete this post.