Best steam groups?

deals/discounts

Unless you're referring to non-PC games, that'd be kinda redundant since Steam sends you an e-mail whenever a wish-listed game goes on sale (on top of having those announcements pop up every time you open the launcher).

updates/news from the gaming world

and while that makes more sense, I'd think that topic would be better covered by various YouTube channels. Personally, I follow Jim Sterling, but you could always check out the video game companies' individual channels for their own updates.

In other words, I'm not sure of any Steam groups that'd fit your criteria, but I think my favorites would be BackLog Assassins ExtraOrdinaire and Screen Archer's Guild. Maybe ClemmyGames as well.

Here is a group that sends announcements on deals and also when a game goes free: https://steamcommunity.com/groups/gg_deals

Here are 2 groups from trusted sites that sell games and bundles:

https://steamcommunity.com/groups/wearefanatical

https://steamcommunity.com/groups/HumbleBundleOfficial

I also recommend using isthereanydeal.com, and signing in with your steam account. Whenever a deal on a wishlisted item goes on sale (individually, or in a bundle), you can be emailed about it.

Also, bundleupdates.com is a good one for being notified about bundles via email when they go live, regardless of whether you want anything in that particular bundle, so it doesn't rely on your wishlist to send email updates.