1) You can now add markets anyone has created to a Serum DEX GUI!
dex.projectserum.com
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2) There isn't yet a GUI to add a new market -- you have to do it command line -- but there will be a GUI soon :)
You can test it out with:
Market Id: 637SgeEW6yFUhk9kC38JJQwWMd9eQJLW9aSkQP8ntZpm
Program Id: EUqojwWA2rd19FZrzeBncJsm38Jm1hEhE3zsmX3bRc2o