Non-derogatory word for shill?

Solution 1:

In psychological experiments, where you have actual subjects and people who the subject is meant to think are also subjects but are secretly working with the experimenters to influence them, I have seen the word "confederate" used.

https://www.alleydog.com/glossary/definition.php?term=Confederate

There are plenty of words that could generally be a positive version of "shill" (like "advocate"), but in the specific context you suggest I'm not sure there are that many, since it's a scenario of deliberate deception. Which is usually considered a bad thing.

Solution 2:

Rent-a-crowd [plural rent-a-crowds]

A group of people who are paid to attend an event to increase attendance figures, rather than attending of their own volition.

The term is derived from a fictional company, Rentacrowd Ltd., mentioned in the Peter Simple (Michael Wharton) columns in the UK Daily Telegraph newspaper.

Wiktionary

...

rent-a-crowd (also rent-a-mob) NOUN British informal

A group of people regarded as regularly or instantly available for an expedient purpose, especially a public demonstration.

ODO

While rent-a-mob is obviously pejorative, rent-a-crowd can come across as being more humorous.

Solution 3:

If you don't mind using two words, "Paid Attendee" is probably the clearest neutral way to convey the concept.

Solution 4:

Claqueur.

https://en.wiktionary.org/wiki/claqueur

Originally a person who was hired to applaud at a theatre performance. More generally, a person hired to create "enthusiastic buzz" at an event, as you put it.

Solution 5:

Promoter, one who is paid to promote an event or company.