shawfactor

Allows Buddypress group admins to send email and private messages to all group members from the group admin/manage section.
This plugin sends each message (PM or email) individually to each group member. And supports merge tags to personalise the message for each recipient.
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LH Buddypress Email or Message Group Members

Allows Buddypress group Admins to send email and/or a private message to all group members .

alexhal

Allows Buddypress group Mods to send email to all group members from group admin/manage section. 
To enable E-mail members for mods simply put the code given below in your bp-groups-members-mail.php file in the end from wp-admin-> plugins-> editor :
<pre><code>add_filter('gmm_enable_for_mods','gmm_enabled_for_mods');
function gmm_enabled_for_mods(){
 return true;
}
</code></pre>
By default this “E-Mail Members” section is available to mods and admin . 
To change it a filter is available and you can change the access like this : 
To restrict this option to admins only 
 add_filter(‘bp_gmm_auhority’,function(){ 
 return ‘admin’; 
 });

Group Members Mail Plugin

I've done a quick test and it worked with no problems for unformatted email

Does what it says on the tin!

Allows Buddypress group Mods to send email to all group members .

LH Buddypress Email or Message Group Members vs Group Members Mail Plugin