RewriteRule ^group([0-9]+)$ groups/index.php?id=$1 [L, QSA]
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^group([0-9]+)$ index.php?id=$1 [QSA]
</IfModule>