The Online User block is only available to users of a FLO collaboration space
The Online user block shows a list of users who have been (or currently) logged into the collaboration space. The list is updated on a regular basis and the default is set to show every 5 minutes (this time limit can be changed to suit). To send a message is very similar to sending a message via the messages block.
1. In your collaboration space locate the Online user block
2. Click on the speech bubble or the person's name