Instructions from Facebook on how to send messages to entire group.
How do I send a message to everyone in my group?
Any group member or admin can make posts in a group using the options at the top of the group. When a post is made, members are notified on Facebook and by email, and they can respond with comments. Members can change their notification settings, but they will always see new posts when they visit the group.
Group admins can’t send private messages to all members of a group. The best way to reach all members at once and start a discussion is to make a post in the group.
If this doesn't solve the problem go to Solution 2
This is going to require a little coding and some Facebook Open Graph knowledge.
You must be logged into facebook :)
First you will need to query all the members of your group. Here is an example query
This only works on groups you are a member of. Also my example link is set to take the first 50. If you need more adjust the Url query accordingly. The important number in this is the gid. Your gid can be found in the url of the group page.
Now you have all the uid (user ids)
These uid's can be used to get their user account names Example:
Here is Brad's
Take the account names and append
Create a csv with all the email addresses copy this line into Facebook messenger.
You would want to automate this in a script, as manually looking up each individual user name would be tedious.