You have to go into the user profile in the ACP and promote the person. You can only do them one at a time. Thats as helpful as I can be. Ive not seen the inside of the invisionfree ACP.
The banners are probably some form of advertising script. When you get out of invisionfree, you can do some searching on advertising scripts, until then, I believe invisionfree will not allow you to advertise.
If you are serious about having a forum website, I recommend you get away from invisionfree as soon as possible. Otherwise all the content you build up while there will be lost. There are a bunch of free forum softwares and even more cheap hosting services. Spring for the $5 a month and $8 a year to have your own website/domain. That way, if you actually find something that someone wants to be part of, its yours.