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For encouraging membership I would create a block and set the permissions so only anonymous visitors will see it. If you use HTML, centre-centre blocks can be nicely designed with text and images.
As far as content is concerned, many modules allow you to set category-level permissions and some allow article-level permissions. This means you could easily create a selection of sample articles that anonymous (and registered) users can see.
Regardless of the permissions you set, some modules will still display a list of prohibited items in their 'recent items block', however anon users will not be able to open them.
This isn't such a bad thing really, as it shows people what they are missing by not registering.
The XOOPS permissions system is a bit tricky to get the hang of, but it is very powerful and one of the features that sets it apart from other CMS solutions. Searching the forum and the FAQ should reveal all.
A thread is for life. Not just for Christmas.