There are situations where it would be helpful to have people buzzing around enthusiastically. Examples would be an open house or a book signing. What would you call people hired to attend such events? Shill seems too harsh.
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.
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.
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.
While rent-a-mob is obviously pejorative, rent-a-crowd can come across as being more humorous.
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.
Most of the following related terms are similarly pejorative, except the last: stooges stool pigeons flunkeys flim-flams dupes snitches chisellers grifters welchers warm-ups (as in a warm-up act before the main act, often used before live comedy shows on TV to get the audience in the mood)
Alternatively, you could make up your own term based on a related concept, but the manipulative nature of the objective means it will be difficult to avoid the implication of deception. Some examples (gratis): Fob-mob foxers stingers wringers diddlers