Hi Kaz,
There’s a reason using forums posting to build SEO value. It usually depends on the nature of member posting in your forum. If they are offering value to the users with their great ideas, it is a good idea for you to help them offering one or two do follow backlinks. However, take a time to think of it this way, if they're black hat webmasters abused by several visitors over the past several years. These spammers definitely love to go around every web forum they can leave and build their links. In case if your forum is active, then you can review it for the approval. But if the forum is relatively inactive, nothing will happen to the link instead it would sit in a passive way and gain some minor SEO value. The web forums themselves weren’t black hat or spam, so they still had positive PageRank to pass to the spammer’s site.
As we all know that nofollow links, obviously, don’t pass PageRank and aren’t that valuable to SEO. However, it is also essential for you to convey to your member that forum links will do nothing much for increasing SEO value. I can understand in many cases the owner of the forum board like you have set them to nofollow. Even if you've not put, Google cares for it in a vast majority of web forums, if users might find their information much useful.
In using web forums for website benefit gaining some more boost in search engines perspective is to realize that they aren’t supposed to be treated like websites boosting or blog comment sections rather they need to be considered like social networks offering great value to the audiences.
Hope you will find it useful.
