Greetings Frank! I too am receiving the same error upon installation:
Warning: join() [function.join]: Invalid arguments passed in /home/xxxx/xxx/wp-content/plugins/wp-maintenance-mode/inc/wp-maintenance-mode-settings.php on line 228
There is nothing in the Exclude field; it's blank. I have uninstalled and reinstalled your plugin. I disabled a few other plugins (Bulletproof Security for one) and it didn't make any difference. What's odd is I have used this plugin on other sites and it has worked fine.
In the warning message (which appears directly above the Exclude field) the words "function.join" are a hyperlink to a "page not found": http://site-address.com/wp-admin/function.join
If I check my preferences and click 'save', the Preview button displays the same "page not found" error. Odd. Hope you can figure this out...