wp-signup.php has a warning message

When I go to my wp-signup.php page I get this message

Warning: Cannot modify header information - headers already sent by (output started at path.../bp-community/header.php:4) in /homepages/path.../wp-includes/pluggable.php on line 865

I get the same message twice.

My network is here: http://designstyleguide.net/community

How do I fix it?

Thanks,

Kristina