Send email to all users in one subsite

Hello,

I have a single-Wordpress install, with Buddypress. I want to send an email to all users.

Right now, I have installed "BuddyPress Group Email" that allows me to send mails to users, in a group-by-group basis.

But I have lots of groups, and I need to contact them all at once.

Please, how can I do that?

Thanks in advance.