How would the browser differentiate between them?

Wish I knew. not a great coder. Cookies perhaps? Or, maybe I could use secondary groups.

It uses cookies now to see if it can get a userid so it can get the users usergroup. If it does not find the cookie then it uses a default usergroup that you set.

How does the RSS feed check for cookies? Does this mean you need to use a browser for reading the feeds? Or can all RSS readers store cookies? Please pardon my ignorance.

Also, a bit off topic here but is there any way to set the RSS feed to send all new posts instead of just new threads?

Dont know about the 2nd question.

As for your first, it will only work if you use a browser I guess like in FF or IE. If it can't get a cookie then it will use the default usergroup you set.

I don't want to show any rss feed for guests or regular members but only for a special usergroup.

Is that possible?

Yes. Set it to look at cookies. Also, set the usergroup setting to usergroup 1 which is guests.

The thing is that I want only one usergroup to get the feed, not any other logged in user or guest.

Then you have to play with the hack your self.

make an IF around the hack

Could you give me a small advice about how to do that? I am not quite sure what you are talking about.

I already tried to edit the postbit_external though but the usergroup conditional seems not to work there as saving is not possible without producing syntax errors.