You can show your availability by picking one of these 4 statuses: Online, Busy, Away and Offline. 

You can change your status by clicking on your name at the top right corner of the screen. Each status has its own colour. The ring around a profile picture changes its colour depending on the user’s status.

Green means the user is Online, red means he/she is Busy, orange means the user is Away and gray means the user is Offline.

As such, when you have set your status to ‘Online’, your contacts can see you are available. If you have never changed your status yourself, it will be ‘Online’ by default.

Note: if you have changed your status to Busy or Offline, your mobile devices will no longer receive notifications.