If your multisite network is redirecting to www.example.com/wp-signup.php?new=example.com then your site is most likely interpreting www. to be a new site on your network. You’ll need to add some corrective instruction to your .htaccess file. Note: For subdomains, each subdomain will need its own redirect instructions.

# DOMAIN #1 - Force www
RewriteCond %{HTTP_HOST} ^domain1.com
RewriteRule ^(.*)$ http://www.domain1.com/$1 [R=301,L]

You can also add the following definition to your wp-config.php file but any internal page links that do not include www. will redirect to the home page without the above .htaccess code included.

define('NOBLOGREDIRECT', 'http://www.example.com');