I know this answer is a long time coming but for anyone that runs into this later in Godot 3.1, hopefully this will help.
You have to do a typecast on the body parameter to Node (is node?) and then on yes you can do Node.isingroup() which let's you specify the group name and will bass on the true/false result to a condition, which you can then act upon accordingly.
Here's what that looks like:
