Need help with simple rewrite rule (shouldn’t this be easy?)

not sure if this is a problem for you still or not, but if so I noticed a few issues. First off, your domain wasn’t set to be case-insensitive [NC]. Second, your RewriteRule regex has a character class ending with a dash. This needs to start with a dash. Third, your rule sends the request to index.php, which means index.php needs to know how to handle the request. If you want to redirect, that’s different. Give this a whirl and let me know how you make out:

RewriteCond %{HTTP_HOST} ^domainXYZ\.com [NC]
RewriteCond %{REQUEST_FILENAME} !wp-admin
RewriteCond %{REQUEST_FILENAME} !activity
RewriteCond %{REQUEST_FILENAME} !members
RewriteCond %{REQUEST_FILENAME} !groups
RewriteRule ^([^/]+)/?$ http://domainXYZ\.com [L,R=301]

Cheers~