there are a lot of considerations to make prior to attempting a newsletter.
Questions to ask yourself early on include:
1. Will the newsletter be offered as:
a/ a web page document
b/ an email
2. If it will be offered by email you need to offer
a/ plain text email
b/ html email
If it will be offered by email you also need a system in place which will allow you, or the site administrator, the options for site visitors/members to subscribe and unsubscribe to your newsletter, choose their prefered format and also to allow you to post the newsletter to all subscribers in the format they requested.
It can all be get quite complicated and whenever I do a newsletter type web application I build as much into the back end as possible to allow for as many eventualities as I can.
There's an interesting link click here with other links to sites that may be useful.
You mention that the newsletter will be to forum members, so if you are a capable web programmer you could write a module for your forum or, better yet, check with the forum script originator to see if one is available. Many forum scripts have add-on newsletter support.
Producing the newsletter itself is possibly the least difficult step, although it can become inconvenient to output a specific quantity of interesting information on a regular basis. Preparing and editing the material on a bi-weekly or monthly basis can take out a large chunk of your time and once you start a newsletter it can be difficult to give it up without adversely affecting your memberships view of you/your site.
I'd have good old think about whether the additional workload will add to your site, its quality and appeal and its members use of it.
Newsletters have a lot to offer in the right circumstances but they also involve a lot of work to prepare and send out and can require a quite sophisticated data driven back end system to effectively operate.
Only you know whether this will work to your advantage in the long term and whether you or someone connected with the running of the site has the time and inclination to go for it.