How to send user password during registration?
This doesn't answer the question, but it has to be said.

You should not e-mail users their password.

It's highly irresponsible to do so.

There are strong reasons for Drupals default behaviour.

People tend to re-use their passwords in lots of places, and e-mails are not a secure communications channel.

If your end user re-uses the password for their VPN tunnel, and login to their e-mail over an unencrypted channel, (no https for the e-mail, and no password on the local wifi for example), then you have just broadcasted that password for the world to see. And now you've facilitated the cyber attack on their workplace. There's probably a large number of both known and unknown ways to compromise the passwords as well. NSA are practically guaranteed to get it. 

Don't do it.

Yes, it's the users fault for re-using their password and not requiring HTTPS to their mail and a strong password on the wifi, but you are the developer and need to know better.