tl;dr: add /rss at the end of the base link of a given group

Long version of the post:

Usually you get the feed by going to the announcements tab of a given group and clicking the "Subscribe to RSS Feed" button or copying its embedded link, but some groups use a different interface where the button is not there.

However, it's still possible to get the RSS feed for a group without the button.

Taking example Top Hat Studios' official group, no button at the announcements tab is given, nor any option to change views, and no links at the page's source. But adding /rss to the link of the overview/home/landing tab gives you the link the button in other groups would give:
https://steamcommunity.com/groups/tophatstudiosofficial/rss