There's a lot of ways to make money with a forum. You can make money by creating a marketplace where members can pay to post their offers, products and services.
You can make money with sticky posts, where members can get their posts to show at the top of a specific forum section for 30 days. You can create a membership site, whereby members pay a monthly or annual fee for a newsletter and monthly exclusive content. You can also add a biweekly webinar or google hangout for subscribers.
You can create a private section whereby the website/forum owners offer training and hidden forum posts/threads. Where members exchange in-depth ideas with some proof of success, anything that will never be available to ordinary members or general public. This can be a one time fee.
You can also sell t-shirts, mugs, caps, hoody etc. with the forum brand. This will also create word of mouth and bring tons of traffic.
Another way is to create events, when you have lots of members then you can create an event for them and charge entrance fee.
There's tons of ways you can make money with a forum, the best part is that you can implement everything at the same time.
Hope that answers your question. But if you're going to join other people's forums then you'll have to do the usual: offer value, have an offer in the signature, buy sticky posts, advertise in the marketplace, buy banner placements, text ads, bribe the most active users to place your offer in their signature etc.
