The way WhatsAgent works is simple: enter the phone number you want to track. Then just sit back and wait. Every time this phone number goes active on WhatsApp, you'll get a notification. And not just that – the app also creates a full connection report.
As if that weren't enough, WhatsAgent also lets you compare the connection times for two different numbers so you can find out how often they overlap, if they tend to connect or disconnect at the same time, etc.
WhatsAgent is an app that should be used with caution. As with all these apps designed to spy on people, you need to seriously consider whether you'd like anybody else to use the same app with you. Use with discretion.
Requires Android 4.1 or higher