As you know, registration is a real barrier for conversations, many of people like to join a discussion but when they try to do it they are stoped by registration process !!
To get the maximum conversion rates for people interested in a content the ideal is to let them post the content and then redirect them to registration, 99% ll do as they want loose content they already spent time to add it.
Here an example of process made by IPS Board with is a forum script (i use it for another website) :
Posting as a guest :
Registration form after posting as a guest :
Email reminding user to finish registering :
All details in the website :