I have signed up for the mailing list, but I havent "received" the thread Id like to reply to.
Hi Qube Newb, you will not easili be able to reply to any email threads that happened before you signed up, as as you noted you will not have receive them.
I do not see an easy way of requesting old email from google groups so in my mind there are two option, both having some downsides
1. reply via the google groups page (https://groups.google.com/forum/#!forum/qubes-users) AFAIK this requires you to have a google account to reply from there
2. You could copy the contents / subject of email you want to reply to from the web interface of google groups. This will break email threading and as a result it will not be seen as part of the same conversation by the google groups web interface and not grouped with the other emails in the conversation, similarly any email client that does a threading view will have it separate from the original thread.
Maybe others have better suggestions