From Xojo Documentation
Registers GroupName as an identifier for messages sent by SendMessageToGroup.
GroupName can be any string, which will be mapped it to a valid Class D IP address. Class D IP addresses are in the range: [220.127.116.11 to 18.104.22.168]. For example, you can call Register("My Cool App"), and it will be hashed to a valid Class D IP address. However, if you use a valid Class D IP address, it will use that without modifications.
Before calling Register, please set the port and call Connect.
This example registers the user as a group member and updates the list. Everyone on the network who wants to join the group needs to use the same group name: