A quick look makes it look like there’s no easy way to get data from Yahoo! Groups. (There used to be a way, but it broke in 2013.)
If you’ve got an MBOX file with the archive, one could use the mbox import script. If you don’t, you could start building one now and do the import after a month or so.
Your next best bet might be to post to the group and ask for a set of early adopters and get them on board with the new site, then do a bulk invite.