I'm trying to figure out how to make it so that my member-based social network site can be spidered but is not possible for guests to post or interact with content in any way. What is the best way to do this? If anyone can suggest an article or tutorial specifically for this that would be awesome. The system I've had up in the past was a check user log script but that was for the whole page, is there a way to do this for (as an example) clicking inside a text box to type a comment or something?
If your member based content is behind a login then you've already got most of what you need to do this in place. When a user authenticates themselves on your website you probably set some sort of flag (probably in a session variable) to flag them as authorized so they can move freely throughout the member's area. At this point in time you can also set a flag that gives them write permission in your website. Then users you allow to view this content because they found it through Google won't have the necessary permission to interact with your content because they will not have authenticated themselves.
You can also make it so anyone trying to submit a form, or otherwise modify your content, is rejected because they are not logged in. In fact, hiding the necessary tools for adding or modifying content to anyone who is not logged in is probably the smartest way to go about it (although for security purposes you also want to verify that all submitted content is from a logged in user).