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.