To invite a family member to join you on WikiTree simply include an e-mail address when you create their profile. After you submit the form you'll be offered the opportunity to add them to Trusted Lists of family member profiles.
If they already have a profile click the Edit tab on their page. Then enter an e-mail address. This will automatically send an invitation.
More information can be found at Help:Invitations.
To share a Family Tree, try this link. You also have the option to share to Facebook, or you can directly email the link along with an image of the tree.