Heya!

I'd like to let any user be able to add more participants in PMs, not just power users and people who started the pm. Is this possible/How?

In messages.template.php, find:

                  if($user->id == $starter || $user->power)
                  {
                        echo '
                        <div class="flex01">
                        <a class="btn primary btn-sm" href="#" data-toggle="modal" data-target="#newparticipant" title="',$text->button->add,'">
                              <span class="fas fa-plus" aria-hidden="true"></span>
                        </a>
                        </div>';
                  }

And then just remove the if statement around it.