tldr: chapo.chat isn't ready yet.
Don't worry, you'll get to be called libs by the users from lemmygrad.ml Soon™.
[chapo.chat is] running a heavily modified version of lemmy forked months ago, for not just the front end, but the back end as well. Once they’re up to date with the most recent lemmy versions that have federation working, then we can try federation, on the back end at least. Their back end devs are great, but their front end devs contribute nothing upstream, and will have to re-create all the federation-related parts of the lemmy-ui code for their own fork.
EDIT: See @Beatnik's comment
How would interacting work, I don't really get it.
You could subscribe to communities on other Lemmy sites and have them show up on your homepage. You could talk to users on other sites using your chapo account. Other users could do the same and e.g. a vegan lemmy user, who doesn't use this site, could still get !vegan@hexbear.net posts appearing on their homepage.