Here's how to configure your Xbox One to display popup notification each time one of your friends comes online (sign-in to Xbox Live on their console). The procedure is as follows:
Press the menu key of the controller. Go to Settings > Preferences and then select the Sign-in & broadcasts menu. You have the choice between three different settings:
No one - to disable sign-in & broadcasts notifications.
All friends - display notifications when any of your friends come online, which can be quite annoying when you have a lot of friends.
Favourites only - enable this setting for your favourite friends only (Go to your Friends page, select a friend, press the Menu button and then select the Add to favourites).