Learn English – What you call someone who leaks information from a team

If they are participating in a scientific research study, an individual would be called a participant and a group would be called a test group.

If the group is surveyed without an actual manipulation, it would be described as a focus group. The individuals would still be called participants.

Since you've edited your question, it seems you want John's friend to use a word that implies that John is just a pawn. Rather than something that is specifically about research participation, I'd recommend one of my favorites, patsy, which is insulting and implies that John is mindlessly and foolishly going along with whatever he is told to do.

If you want a phrase that is patently offensive, you could use meat puppet.

Perhaps self-absorbed

absorbed in one's own thoughts, activities, or interests

or egocentric

regarding everything only in relation to oneself; self-centered; selfish

or self-centered

concerned solely with one's own desires, needs, or interests

SUPPLEMENT

Also consider self-directed

(of persons) free from external control and constraint in e.g. action and judgment

Note that this term is often applied to someone who does attend to the needs of other but exerts a good deal of independence in doing so.

and autonomous

Not controlled by others or by outside forces; independent