BuddyPress.org

Do you have the following line in your wp-config.php or bp-custom.php?
`define( ‘BP_ENABLE_MULTIBLOG’, true );`

If so, I suggest you comment it out:
`// define( ‘BP_ENABLE_MULTIBLOG’, true );`

BP multiblog mode is only necessary if you want to have BP content (like members profiles, etc) available on all subsites. Regular WP multisite use does not require BP_ENABLE_MULTIBLOG.

If you absolutely need the BP_ENABLE_MULTIBLOG functionality, but you want to force all registration to happen on the main blog, then you will have to build your own redirects.