I use and like hashtags on my group's messages. I don't need hashtags on group notifications, but I can live with them if necessary (I'll just ignore them).
What would be infinitely more useful for group notifications is a change to the subject lines for mods who receive notifications via e-mail. Here's an example of what I mean:
Somebody applies to join my group. I receive an e-mail:
Subscription Approval Needed - email@example.com applied to firstname.lastname@example.org
That's absolutely fine. Works great for me. Amongst other things, I can sort applications alphabetically if i want to.
One of the other mods approves them, often while I'm asleep. I get:
New Subscriber To email@example.com
I may get up in the morning to five or ten of these. I have to plough through my inbox matching the applications with the approvals. How much easier would it be to see:
firstname.lastname@example.org has joined email@example.com
Similarly if somebody leaves I get:
Subscriber has left firstname.lastname@example.org
How much easier to get:
email@example.com has left firstname.lastname@example.org
Thank you for your consideration.