Continuing the discussion from How do you add another person to a private message when it's already sent?:
Wanted to post a reply in the other topic as that topic was the result Google offered but it was closed.
Needed this option again today but could not find “Add or Remove..”, however the “+” is still there under the first post.
Clicking “+”
Are you suggesting that the ‘Add or Remove’ button be moved to the bottom of the PM instead of just below the OP?
The extra step to click “add or remove” was removed in
Thanks for asking.
No.
It appears that some of us are conditioned to assume that familiar mechanisms—such as using “@” mentions or similar methods to invite users into a topic—also apply to private messages. In practice, that is not the case; adding users requires using the “+” control.
In the earlier, now-closed topic from a few years ago, I referenced the “Add or Remove” button. When rereading my reply today, I went looking for that button and found it was no longer present. However, the note about checking under the first reply was still useful: the “+” control is located there, and it works as expected.
As a result, my reply from years ago is no longer accurate. I am posting this clarification for others who may encounter that topic and wonder why searching the page for “add” no longer locates the control.
I am not expecting the behavior to be reverted, though it is admittedly easier to search for “add” than for “+”. 
