By default, you should get a notification on your iOS or Android device for every new message. Sometimes it's easy to disable this, but by default, notifications are on.
If they are disabled, you can enable them from the system settings:
For iOS, you can follow the instructions here (Scroll down to "If you don't see notifications for a specific app").
For Android, you can follow the instructions here.
You can also enable notifications via mail. You will then get an email with a short excerpt and a link to the conversation for every new message in a conversation.
This can be configured from the Notifications section in your Profile.
Alternatively this can be changed by an administrator in the user management section.