An Azure relational database service.
Creating and connecting to an Azure database is the easy part. There are abundant resources about that. For one thing, most anything showing how to connect to a SQL Server database can be used to connect to an Azure database; you simply use an appropriate connection string.
The hard part is keeping track of who is using the app. Is this one system executing many instances of the application? Probably not. I assume it is many systems, each executing separate instances. So you need to have a way for them to communicate. You can do that using a peer-to-peer mechanism (which would be too complicated) or you can create a server that each of them communicates with. Where will that server be? You can create a service in Azure that provides that.