If you've included the people already on WikiTree who you want to link to in your GEDCOM, make sure that their entries in GEDCOMpare are matched to their WikiTree profile -- usually GEDCOMpare finds them and suggests the matches, but sometimes it misses them, especially if they have a common LNAB. You can click Compare With in the right-hand column and enter the profile ID.
Once you've linked the existing person (and once you've completed all the matching and rejecting of GEDCOMpare's suggested matches and are seeing the Add buttons), you can click the Compare button and start adding their family members from that screen.
