Since the earlier response to your question does not seem to have been on target -
From "Family Trees" in the top bar, hover or click to access "More..."
If you have only one tree - click on Manage tree. If you have multiple trees, click on Manage Tree under the tree you want to share.
Once that tree displays - click on "Sharing" then on "Invite People" on the next page that displays. You can invite by email address, username or other methods.
Hope this helps.