You could keep track by keeping their names on signs or I believe there is a command, but I'm not sure you'll have to ask Melgrath
/is ? Tells you commands I think. If not, does /is party work? I'm a bit rusty with these commands and I don't have access to my PC to try it out. Ask Mel.
Typically /is party is the best way to see who all is on your island. That being said however, there is an issue with the Skyblock plugin that sometimes this command does not work, or does not properly show island members. I have reported the issue with the commands, but do not have a timeline to when it will be fixed. Thanks, Melgrath
Closing since your questions has been answered. If it has not please feel free to make another thread