How do I create a custom search function to only show contact information if they search for an exact unique identifier?

Here is one way I can think to do this.

1. Make a new post for each attendee. The post title should be their unique identifier that you assign them. Rather than manually creating all those posts, you might find it easier to create a spreadsheet and import. Each posts’ content would be the contact info of the attendee.
2. You can password protect the posts through the visibility setting on the edit post screen. That way the contact info isn’t public. Maybe give all the posts the same password and then share it at the conference. If you decide to go the import route to create your posts, the password will be one column in the import spreadsheet.
3. Attendees can search for each others’ identifiers through the regular search functionality of your site, whether that’s a dedicated search page or a widget. People can even type in yourdomain.com/?s=uniqueidentifier into their address bar, which will automatically run a search for “uniqueidentifier”, although in that case make sure the identifiers don’t have any characters that need to be url-encoded, like spaces or apostrophes.
4. Password protected posts still show up in search results. People will be able to click the post in the results, type in the password, and view the person’s contact info.

One caveat to this method would be that if the identifier is used elsewhere on your site, for instance in the content of another post, then both posts will come up in the search results. Only one of them will have the identifier as its title, but it still would be confusing. Another issue would be that the titles of your protected posts will still show up on your blog page if you have one.

However if you choose distinctive identifiers unrelated to the normal content on your site, and/or you don’t really use your blog page, this could work for you.