Since my forum is a bit inactive, I want to make it more attractive for other guests to join. Which is why I want to have like 3 or 4 guest on the who's online list 24/7. How do I do this?
Do not do this. This is the worst thing you can do.
Advertise your site, make content, and give users something worth registering for. Faking your traffic stats is a no no.
Well I have done everything in the last months and it's not helping. I have a twitter account for my site with 200 followers, 34 likes on FB, a YT account with almost 15.000 video views. I have lots of backlinks and content gets posted daily. There are only 4 active members inluding me and I have like 55 members, 20 of them used to post but they just started to boycott my site because my competition made a deal with them. I only have 2 sites to compete with. My site was also on the first page of google and I got a lot of guest, it's on 2nd page now with less guests but yeah.
So I think it's because people see 0 people online they don't want to register
People don't go straight to the index page to see who's online. They will usually find a topic via Google, read it, and if they have something to contribute they will register and reply.
After that, they'll either post some more or never come back. It could be either, but it all depends on what you have to offer.
Edit : not seen above two responses
I do not remember seeing any free plugin for your required purpose ...
first post here might help