Unified logins

Unified logins

I  have used zoho as my primary email for at least the last six years, but it wasn't until recently I became confused with everything.

My girlfriend's family is spread out from Canada to Italy, from the UK to South Africa, and I am working on setting up owncloud to bring them all together.  each family member gets an email, yada yada, but I can't figure out a way to unify logins, any experiences or tips?