To merge your tree with that of your family members, you must go to your tree and then click on the box of the person you want to merge your tree.
A new window will then open where you can invite them to join the family tree by clicking the "Invite this person" button (1):
You can then add their email address (1) and a message accompanying the invitation (2):
Once completed, validate the invite. A merger request will be sent to them to accept or reject.