Have your association's site answer on its own address, such as www.yourclass.org, instead of a we-muster.com one.
An association's site can answer on its own address, such as www.yourclass.org, with the address staying in the visitor's browser bar.
We set this up with you by hand. Get in touch with the address you want to use. The only thing needed on your side is one DNS record, added by whoever looks after your domain, and we tell you exactly what it is.
Once it is live, your own address is the one search engines list, and links to the old we-muster.com address still work.