Some times when I come to the forum I see 4, 6 or even 10 people on the site, however they are guests. What is your suggestion to get them to register and post something?

It can be assumed that most of the guests looking at this website are interested in ambient music or environmentalism. So in order for people to register to this website, they need to find what they're looking for.
Of course, a knowledge of the people behind this website and what they do and what it's for is essential, and luckily we already have that. What else could be useful is a greater emphasis on joining the website. Right now on the front page I see a small 'register' option on the top left of the page. Expanding and redesigning it to become more noticable might be a benefit.
Another idea: a highly noticable link on the front page that can be seen and recognised easily could lead to a page with 'Reasons why to join us' or 'The benefits and advantages of joining us'. Therefore visitors can get to read and understand exactly what is on offer here, and why they might wish to register.
Hope this helps.

Thanks a lot. Actually the register option can not be enlarged, since this is a free forum. If I paid, probably it could be done. Reason why to join, people know why to join. If they like ambient music and environment topics, they would most likely join.