How do I prevent everyone from knowing I joined a Facebook group?

[theaceofbass]

I've been invited to join a group and I have to join it, but I don't want anyone (that includes my friends and any Facebook user) to know. Does anyone know how to do this? Step by step instructions would be very helpful. Thanks.
In response to Vai: no, changing your mini-feed does not affect what everyone sees
In response to Dana F: it's my responsibilty to join - it's just so embarassing tho

[Vai]

go to your profile and where it says that you joined it delete it. that way you can join the group without advertising!

[Dana F]

why would you want to hide that you joined a group on facebook step by step instructions dont join the group that way they will be ntthing to hide!!!!!