Is there a way to set up a default hashtag for a sub-group. ?I searched on it but came up #empty.
No, but I can see where a default one can be useful for (sub)groups, i.e. automatically (added if not there) during message posting, if that's what your intent is and not the outgoing messages in which case Bruce showed you the way.
?
If it is something you could really use, you can post a #suggestion message on beta.
Cheers,
Christos
Christos Psarras