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 - xyz@abc.com applied to support@tanyackd.groups.io 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 support@tanyackd.groups.io
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: xyz@abc.com has joined support@tanyackd.groups.io
Similarly if somebody leaves I get: Subscriber has left support@tanyackd.groups.io
How much easier to get: xyz@abc.com has left support@tanyackd.groups.io