It is possible that someone in your family has joined you on Famicity but it is not added to your tree automatically. This happens when they created their account without an invitation via your own Famicity page.
Do not worry, you can invite them to join your tree afterwards.
Just click on the box in your family tree, then "Invite this person" (1) in the left menu:
A new window will then open:
You can then add their email address (1) and a message to accompany your invitation (2).
Once completed, an invitation to join the tree will be sent.