Invite them using the same email address they used on their other tree
Here is what they will do:
Receive a request to join your tree (the same request will indicate that you are asking to merge their tree with yours)
They will accept the request
Then once the trees have been merged:
Everyone in the shared tree can assist with merging all duplicate profiles (removal of these duplicates will result is loss of lots of hard work)
Drag and drop profiles in the tree
To merge duplicates, use the ‘Move this Person’ button found in the 'more' link of a node on your tree.
This will add the node to the 'Map These People' tray, found on the side of your Tree view.
Then drag the node onto the duplicate person and select "They're the same person"
"Merge This Profile"
You may also manually merge duplicate profiles within your tree and also any duplicate public profile outside of your tree by using the "Merge This Profile" feature under the "Actions" tab. (Tip: View the other profile first to ensure that it appears on your list of recently viewed profiles.)
Go to the profile you want to merge.
Click on "Actions".
Click on "Merge This Profile".
Search for the duplicate by typing in the person's name.
Click on "Compare Profiles".
Compare the profiles if they are the same person, click on "Yes, request to merge". If they are not the same person, click on "No, they're different" or "I'll decide later".
For information regarding Tree Merging, please visit the following Wiki For information regarding Profile Merging, please visit the following Wiki
Note: As of November 28, 2012, merging is free! Please note Tree Matches are still a Pro feature.