There’s no admin feature to change the text of the activation email, but you can apply a filter to it.
Put this in your theme’s functions.php:
function my_activation_email($message) {
$message = str_replace('Thanks for registering! To complete the activation of your account please click the following link:','My message is the best message ever.
Really it is.
So I guess you should activate your account now.', $message);
return $message;
}
add_filter('bp_core_activation_signup_user_notification_message','my_activation_email');
This should so become a WP plugin or bpress addon or something.
For some really odd reason this is not working for me.