Answer to your main question:
I want to know if it is possible to call a friend who has another Google Home
is no, you cannot do that exactly. In the link by @Bence Kaulics in comments it is said you can call outbound (to a phone) but inbound calls are not possible. That means the destination cannot be another Google Home but only owners phone.
Making an outbound call is as easy as saying Hello Google, call NN, where NN is one of your contact or a phone number.
At the moment though 911 calls aren't possible. Another issue is that for normal user your number is not shown to the responding phone, which may feel embarrassing.