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All existing users can invite anyone and their username will be their referral code. New users who register/join with your referral code/name give you chance to earn 100 ESTM each, once they earn 250 ESTM

Mh, not sure if I understood.

I can invite existing and new steem users, and once they reach 250 ESTM we both get a bonus? Are there click/signup stats somewhere? :)

Existing users don't need to be invited. It is about new users signing up for now, inviting existing users is next iteration, it is tricky to track for existing users without adding extra field on login...
ps. referrer gets a bonus.

No you can invite new users to esteem...whether the person is a new or existing steem member, as long as he/she doesn't have an esteem account