Yes, it is possible to tag a Facebook group in a post. Below are instructions:
Log into your Facebook account and then navigate to the item you'd like to update with a tag. Click on the "Comment" hyperlink beneath a status or image posting to tag a Facebook user in the post's comments sections. To tag a group to your status, click inside of your status box at the top of your news feed.
Type an "@" into the comment or status box, followed by the group's name without a space before it. For example, type "@soccerclub" to tag a group named "Soccer Club" to your post. Facebook will attempt to predict the group for which you're searching. Click on the group's name when it appears beneath the comment or status box.
Click on the "Post" button to submit your status update with the desired group tagged in it. For comments, hit the "Enter" key on your keyboard to post your comment and its tag.