MineFriends will let you know when your friends are online. You need the server plugin as well as the Android mobile app.
Just add your friends and after they accept, you will be able to see when each other log on or log off any server running the MineFriends server plugin.
Android Download Link:
iOS Download Link
You can only see friends who have accepted your friend requests. If you deny a friend request then the person who invited you cannot see when you are online. You can also tell MineFriends to "Hide" you so that your logons do not show up to your friends. This is handy when you want to join servers for a while without letting people know. You can then unhide your self when you want to let people know again (option is in Settings).
- Send and receive friend requests
- Get notifications when you receive friend requests
- Get notifications when your friends log onto servers
- Configurable notifications: Sound and Vibrate settings for each friend