The instructions for uploading a GEDCOM file appear beginning on page 48. It is a multi-step process during which each name is compared to the existing data to avoid entering in duplicates. Quoting from the Reference Manual at page 49:
This GEDCOM upload process has many planned enhancements. Until they are complete,With this reservation, the process of adding names is outlined in detailed instructions. The upload process has several parts:
please be aware of the following issues:
• You cannot currently add the notes, sources, and multimedia links that are in your
GEDCOM file to Family Tree.
• During the upload process, you compare what is in your GEDCOM file to what is
already in Family Tree. If you stop before you finish comparing, simply make a note
of which filter and person you left off on.
• You can upload records of living people, but you cannot currently add them to Family
1. Upload your GEDCOM file, and compare it to Family Tree.You can see that this might take some considerable time, if you try to add a sizable file of names.
2. View the results.
3. Review people who are potential matches.
4. Review people who can be added to Family Tree.
5. (Optional) Review people who are already in Family Tree.
I am highly suspicious of adding this feature to Family Tree because of the danger of adding in duplicate information and duplicate individuals. It is all too easy for people to say that a record is not a match with an existing record because of some small or large variation in the data being submitted. For more details on this procedure, please refer to the Manual.