It tells you - usually - who added the profile. But do not believe it tells you who started the Tree.
Except for when looking at a claimed profile where they joined geni from log-in, not as invite from someone else, thus added their own profile and possibly others - in which case usually you see the line "Added by;" and then after it "Started family tree" followed by a date. -- But since Geni's World Tree is all connected and interspersed - there is not necessarily one person who started any given branch/twig/ tree within the Forest which is the World Tree or etc.
The first profile in Geni was created Friday December 22, 2006 17:51:55 LA time, and technically it is the one who started the World Family Tree.
So, - define tree... https://www.geni.com/worldfamilytree/learn-more
At one point Geni made a major deal of distinguishing between "Forest" and "Tree" --
Not sure if this is the last remaining place where you can still see both terms officially being used or not - but - If you go to "Research" and then to "Share Your Tree" and choose "GEDCOM Export" -- one of the choices is "Forest including connected inlaw trees A Forest export may take several days to complete." -
Jennifer - Yes. Anyone can build out - up to ancestors, and then down from each ancestor to fill in all the cousins - AND build out from spouses of each ancestor and cousin and etc.
Moreover, altho set up for always starting with oneself and building out from there -- a Pro (or Curator) can "cut the tree" -- and some seem to have a great time using that to create, for example, a bogus sibling which they cut off to form the nucleus of a totally unrelated Tree - either because they know it is related somehow, or they think it is and want to build out to see if they can discover the connection -- or because they are big into Projects and adding folks for that purpose.