Not seeing other users or users are dropping
The first thing you need to check is the network adapter settings in SideNotesIM. By default SideNotesIM communicates through the Windows host and may need to be told what network adapter to use in order to communicate on your network correctly.
To do this, click Options then System Option and select the Network tab. Choose Network Adapters and select the network adapter that this computer is using to connect to your network. If you have multiple network adapters you will need to choose the network that the other SideNotesIM clients are using. Be sure and save the settings and then restart SideNotesIM.
If this does not help you will need to verify that you have exceptions in your Firewall and Anti-Malware software. You might also verify that this client is registered and has not reverted to an expired trial.