i came here with this question, and still have it. (thanks yoink, but given the occasional idiosyncrasies of what is accepted, i'd like answer from someone who knows, although your guess is totally reasonable.)

"They are all of my friends" wasn't accepted so I guess it's the former.

Can someone with more expertise clarify? If I say "Sono tutti i miei amici" it means 'They are all my friends,' but it I say "Sono tutti miei amici" (omitting the "i") it means 'They are all of my friends?'

I think, it signifies the second meaning you suggested simply because the first, or an example such as "they are all of my friends" have a slightly different meaning. "All of my friends" and such examples starting with "all of..." might mean excluding the group to which the speaker is referring, there are some other people not present. Yet, perhaps an answer such as "they are all my friends", which to my ear doesn't sound so nice, is more acceptable because this structure simply signify this: "these people are all the people I have got as my friends."

"They" references an unspecified group. All of the members of that group are my friends. eg in response to the question "which members of the group are your friends?" you respond "they are all my friends.

Report it.

No, we can't say it, because we use" both "for two things or people, but there we have more than two.