I heard that if I sign as a developer on fb,I am able to see emails of users
Is it possible if not,what is other way?

You can simply check the info tab of your friend's profile, if they have not set their privacy settings, you can see their email.

Or else, you have to login as a developer/admin as you said. But I'm not sure about it though.

How can I login as dev/admin,please explain me