To use IPS Connect with two or more IPS Community Suite installations, you must first choose which installation will be the master and which installation(s) will be the slave(s). Visit the Admin Control Panel of the master application and navigate to the System tab -> Login Handlers. At the top of the page you will see a notice that tells you credentials you must use in order to notify other IPS Community Suite installations that this is the master.
There is no "setup" required to make a Community Suite installation the master IPS Connect installation.
To set up slave applications in the network, visit the Admin Control Panel of each slave individually and navigate to the same area (System -> Login Handlers). Next to the IPS Connect login method, click on the pencil icon. Here you will be asked for the Master IPS Connect URL and Master IPS Connect Key that you will have retrieved from the master installation previously. You will also be asked if you wish to accept display names or email addresses for the login identifier, or both, and if you want to allow IPS Connect authentications to provide access to the admin control panel if the local user account is marked as an administrator.
After providing the required information and saving this form, you should click the "Disabled" badge in order to enable the IPS Connect login handler, and then you should drag-n-drop the IPS Connect handler to the top of the list to make it the primary login method. This ensures that IPS Connect is checked first before the Community Suite falls back to the local database to validate a login attempt.
Setting up IPS Connect with your Community Suite is complete at this point. You can repeat the above steps on any other installations you wish to connect to your network.