There is no Freshers-WhatsApp group - most of our communication at the institute takes place via the university's own messenger heiCHAT.
Setting it up is a little more complicated than we would like, but it is feasible. You can find instructions here: https://fachschaft.cl.uni-heidelberg.de/en/chat-rooms/
After you have set up the Element app and logged in with your Uni-ID (on the university's own home server), you can join our so-called space (if you use Discord: a space is to heiCHAT what a server is to Discord) and there is then a room for Coli first-years [semester]. There is also the “big” chat room for all semesters, where there is more going on, the “Heimeligen unverschlüsselten Online-Pool”, which you are also welcome to join.