Looking at the clause

"... to talk to two separate people or groups involved in a disagreement"

is the word 'involved' an adjective here? If it is, shouldn't it be 'that are involved'?

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    Does this answer your question? Implicit "which is". Traditional grammar regards the (often totally acceptable) shorter form as a 'reduced relative clause'. Feb 6 '20 at 14:03
  • I think that is it. Thank you! Feb 6 '20 at 14:07
  • There are many posts here addressing the topic; search for "reduced relative clause" (and be aware that some grammars regard the term as inaccurate and explain the shorter forms differently). Feb 6 '20 at 14:09