Is there any way to search for friends using their email addressed?

The only way you can add people to your friends list is if they're current members of subeta. Email, facebook, etc. don't work on here.

I meant like, is there any way I can look up what friends on my contact list are on subeta?

Unfortunately, no

If you are curious about friends playing here, you can always ask on Facebook. It might be a little easier?

Good point! Thank you