[development] How to create a 'registered by' field in the registration form?

[Leo Burd]

Hello there,

How can I create a 'registered by' field to be included on my site's 
registration form?  Ideally, I'd like that field to only accept users who 
are allowed to registered others.  I also would like to show those latter 
users a list of site members that have been registered by them.  Any 
suggestions?

Thanks in advance,

.L.