The Primary Admin of the Tithe.ly account that adds Sites to their plan is the only one that will be able to access the website from within the dashboard using the Sites link located in the left side menu.
How do others else access the website?
Anyone other than the Primary Admin will need to access the website though the website domain and use the login link found in the footer or by adding /login after the domain.
BUT for them to access the site to make changes they'll need to be added to the website as an admin user (or with specific permissions) by the Primary Admin of the Tithe.ly account.
Note: before your website has launched, the domain will a temporary staging domain ending in "tithelysetup.com" (or similar).