I don't believe there is a way to password protect a LR collection made public. Thus everyone with the correct URL will be able to see the page.
Does anyone know if Adobe's server prevent Google and other search engines from crawling and indexing the page?
Lightroom makes it extremely easy to share contents but is there a way to only share to only a few selected friends or family member?
For now my work around is to use Adobe PF (portfolio) and create a password protected page but it is more cumbersome and PF doesn't have an easy way to let people comments the photos (at least not that I am aware of)
I do not think that any search engine will be able to search for the Lightroom collections you've shared with other people.
As of now there is no way to password protect the collection URL's, but it seems like a good idea for a feature request. Could you please share this as a feature request here, so that our engineers can have a look at it for future updates?
A quick search on Google for: site:lightroom.adobe.com shares will reveal that Google indexes many of the privately shared collections. Adobe needs to add a NOINDEX option for the shared albums. This is pretty bad. Even if you don't link to the gallery. Google will index it if someone will visit it using Chrome. That's how they get into the index.