This is fixed on master and 1.x.x. Thanks for reporting.
v1.5.3 Can't mention groups?
We use groups a lot on our forum and I like to use the @Mention to mention groups so everyone in the group is notified. However, I don't see any option to mention groups anymore after upgrading to v1.5.3.
Has this been removed? Or, did I mess a setting up somewhere?
Check the ACP page for the plugin. You should be able to enable group mentions there.