You would add additional IP addresses by adding them as shown below and using the [OR] flag for each additional IP Address you add.
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RewriteCond %{REMOTE_ADDR} !^10\.100\.4\.1$ [OR]
RewriteCond %{REMOTE_ADDR} !^192\.168\.100\.32$ [OR]
RewriteCond %{REMOTE_ADDR} !^88\.55\.66\.200$
RewriteCond %{REQUEST_URI} !^/bp-maintenance\.php$
RewriteCond %{REQUEST_URI} !^/wp-content/plugins/bulletproof-security/abstract-blue-bg\.png$
RewriteRule ^(.*)$ /bp-maintenance.php [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]