Non-derogatory word for shill?

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.

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.

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

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