How do I find the bbpress forum ID for a forum in a buddypress group?

The forum id for a group is stored in the {yourprefix}_bp_groups_groupsmeta table where group_id is the group id and the key is forum_id and the value is a serialized array.

Instead of a shortcode, it may be easier to add a function call in a bp groups template.
Something like: echo gauray_recent_replies( bp_get_group_id() );

function gauray_recent_replies( $group_id ) {
   $forum_id = bbp_get_group_forum_ids( $group_id );
   // collect and return replies
}