By default when you install your board, all users who register will have their online status shown by default. This is controlled by the Default Registration Values in Options -> User registration. Make sure Show online status is ticked.
If some people have hidden their online status through Preferences and you need to enable it for all users again, you can run these queries:
First, enable Show your online status for all users:
SET visible = 1;
Next, enable Show your current activity for all users: