Short Answer: You CANNOT.
'Yes', I received one of those "E-mails" [some two (x2) days ago], it is from a "Campaign" by 'FamilySearch', to increase the participation, by Users/Patrons, in "Family Tree".
My "Email" said:
You have added a family member to your tree. For every family member you add, we’ll start looking for hints—like photos, stories, and more!
[ See My Family Tree ]
WHY do I need congratulating for something I do every day ...
I have added MANY new, Ancestors; and, Extended Family members, to "Family Tree", even of late - nothing new to me.
I did not even bother to look at the e-mail, of two (x2) days ago; but, as you posed this 'Question', I went back and took a look, at that e-mail.
As, I initially stated, you CANNOT find out from that "E-mail" who the "New" Family member was, that 'FamilySearch' is referring to/referencing with that "E-mail", that you (or, another User/Patron) added to "Family Tree" of 'FamilySearch'.
Neither, in (from) the "E-mail"; nor, in the resulting "Link", in "Family Tree" of 'FamilySearch', from the "E-mail".
Plus, unfortunately, from my perspective, that link takes you to the the "Portrait" 'Pedigree' View; as, my preference is the "Landscape" 'Pedigree' View.
The "Recents" was no help; as, I have been working on many individuals/persons, over the last few days. And, there were a number of "New" ones.
So in essence ... again, you cannot.
Just my thoughts.
Just thought I would let you know.
I did go to the "My Contributions" feature/function/facility; and, looked through "My Changes" ...
I went back to two (x2) days ago, right towards the bottom of the list; and, there were some Four (x4) NEW individuals/persons that I "Created"; as to, which one of those four that the "E-mail" was referring to/referencing, I do not know - possibly all four, who knows.
But, which new/inexperienced User/Patron would have know to do that ...
And, if it was ANOTHER User/Patron who added the NEW Family member, there is no easy way to find out; except, the hard way, by examining EVERYTHING ...
It certainly would have been good (ie. better), to know who (or, possibly, even more than one), the "E-mail" was referring to/referencing.
i was afraid of that. thanks for letting me know. makes that email kind of frustrating. lol