Learn English – Word ending in “-ine” for rat-like

As to the first part of your question about finding a word you've forgotten, you could try a reverse dictionary.

I also love, love, love my dog-eared copy of Roget's Thesaurus. (I should probably have looked up some synonyms for "love.") There is something to be said for thumbing through it and finding a category of words to ponder. Serendipity at its best, I tell you. There's an online version of Roget's too, but it's just not the same.

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.