Will the ads manager ever include the advertisement of groups? I understand these advertisements aren’t as popular, but I can’t find any way to advertise a group other than creating a sponsored experience advising people to join.
Or do they already exist - I could just be blind.
Well I’ll answer the second part of your question - unfortunately, no, they don’t.
I’m not sure why roblox removed user ads really - I know it was a cesspit of terrible ads but there was better ways to deal with it (like more moderation) and I dont think that stripping options from people who actually use the service properly was the best move.
For our group, we found sponsoring semi-effective however sponsoring now gives short boosts of players and yields almost no group joins. 
Other than that, tiktok and other social media is kinda effective too.
Maybe roblox had enough of everyone begging for 10k followers? So they are making it harder for people to get this by just paying robux and advertising.
Just a theory though. A lot of people switched to group ads after they couldn’t use user ads.