[wrestlepower]

yes and no. You can add a new profile field that will be required when registering, but i don't think you can make the existing one compulsory. You can always add the user field "referred by" and make that compusory, but in theory they could just type any text in there and it would work and let them register as normal.

I realise it may not ahve been what you were originally looking for, but if somebody is referred by someone else, couldn't you just use the invitation system? If you really want to do it without that, i suggest you make a request in the request forum.