Dziękujecie in plural form

What is the plural form of dziękujecie?

I'm not sure what you mean.

This is plural, 2nd person.

Singular: dziękuję/dziękujesz/dziękuje
Plural: dziękujemy/dziękujecie/dziękują

Dziękujemy and so on means "we thank you", if you look for that.
"Thanks" I'll tranlsate as "dzięki"

Can anyone tell me whether "dzięki" is used only in very casual conversation between friends or whether it is acceptable in more formal conversation?

Thanks (dzięki)!

it is not accepted in formal environment/setting

It sounds somewhat informal, but no one is going to take the offence. I guess. One can say to his boss "Dzięki, panie dyrektorze". At least to me it sounds normal.

if you're in good relations with your boss then it's OK.
But when in doubt, use "dziękuję", just to be on the safe side.

Actually, I didn't for what I wanted correctly. What I really wanted was the past form of the word dziękujecie.

Podziękowaliście

No. It is: dziękowaliście (imperf.). Podziękowaliście is the perfective counterpart.

Cinek