If they are part of the family tree, then you can share it with them by clicking on their name and then clicking "Invite [name] to share family tree". Two notes about this: 1: you can also grant editing rights to the tree, but you don't have to. 2: You can never remove either viewing or editing rights. Even deleting the tree does not remove it for people you have invited.
If they are not members of the tree, you can share the tree with the "Download/export this family..." link in the lower left.
