The totally free ways to get people to the forum would be to get the site rank properly in search engines. This will incorporate a campaign of seo with lots of link building so the community forum may rank nicely in search engines and obtain visitors this way. One more method would be to trawl the internet trying to find emails and contacts of people interested in the particular area of interest of the forum and post all of them an email on launch of the community forum, or even request them to assist you get it started. The usage of Twitter and Facebook to target keyword phrases should not be overlooked. It is possible to deliver your forum RSS feed to Twitter. You can even publish within additional message boards as well as be useful to other members and have a web link to your forum in your signature. Yahoo Answers may also be employed in the same way. Take care not to spam them though. Post with them just like you anticipate people to post on your community forum.

The paid ways to get visitors for the online community is by using Google’s Adwords and Facebook advertisements focusing on the keywords of your niche. You can also buy advertisements on web sites highly relevant to your own niche.

 More on forum traffic and getting traffic

There are many approaches the discussion board manager may recoup this cost and also make some money by monetizing the forum. The simplest and most common way is by using Google’s Adsense. These do appear to do better in some niche categories than others. Another option is to carry eBay feeds of sales that are of meaning to the topic of the community. There are other affiliates you could generate a percentage or referral payment off for carrying the advert. Seek out programs who have goods that are strongly related the discussion boards niche. Carrying advertising for books from Amazon can be useful and so may starting posts to discuss a fresh book strongly related the niche could produce some sales. You can even direct sell advertisements, but want to be familiar with the geographic location that potential advertisers are promoting to. Several only focus on one nation or region and many forums are usually global, so something such as OpenX enables you to geotarget advertisements. The most difficult way to make money using the forum is to have exclusive password protected articles that you charge for use of, but these is only going to work with a large prosperous forum where the public articles is already great and the guarantee of better still articles that is bought.

More:
Monetizing a forum
Make money from a forum
Forum Income

Here is a selection of forums to have a look at to see what the field looks like:
Rolling Stones Forums
Star Trek Forums
Psoriasis Forums
New York Forums
NBA Forums
Frequent Flyers Forums
Legacy of Kain Forums

One of the better ways tro promote a forum is to submit them to Forum directories.

Then search engine optimisation comes in to rank the forum high in the search engines.  You need to submit the forums to forum directories and social bookmark sites. You can also send your forum RSS feed to Twitter.

Once you have the content in place and some good traffic, these visitors then need to be converted to members, then the members need to be converted to regular posters. Then there is the ongoing monitoring of the forum, the management of the staff and the members.