When you change domains it is possible to merge Yammer networks. This allows the old network to exist as an external network you can access. Merges must be requested through a support ticket to Microsoft.
The alternative is to use the Data Export feature to get a ZIP file of all messages in CSV format and files posted.
Here's the recent update from Yammer:
Beginning March 15, 2019, we are rolling out rich text formatting for
Yammer posts. This includes bold, italic, links, bullets, and numbered
Composing messages with rich text formatting is only available on the
web client and Yammer desktop.
Mobile Yammer apps will display formatted messages, ...
You can always tweak your notification settings for your account by going to a URL similar to:
Can you please also provide a bit of background? Is this a paid network? Are you a verified admin? Do you have active directory sync enabled? Are you inviting these users, or are they just signing up?
In Yammer groups, if you post an "Announcement" you get the option to give it some basic formatting, but if you post an "Update" then it's in unformatted text. So, yes, there is a way to do it after all.