Open Site settings → People and add someone by email. They need an account on your installation first. Administrators create accounts under Account settings → Users.
Role
Can do
Owner
Everything, including deleting the site. One per site, and it can’t be changed or removed.
Admin
View stats, change settings, goals, shields, shared links and people
Viewer
View stats only
Roles are per site. Someone can be a viewer on one site and an admin on another.