There are four different user roles in Giving, each with different permissions. When a user is added to the Admin side of Giving, they will be assigned one of these roles: Giving Administrator, Bookkeeper, Counter, or Reviewer.
Check out this clip from Planning Center University.
To add a user, go to the Users tab on the People page, and click Add User.

Search for the person and select them from the list or click create new person.

Notice
If you create a new person, you'll need to fill out their contact information before assigning them to a user role.
Choose the user role, and then click Send Invite Now.

The person will receive a welcome email with instructions on how to access Giving.
You can change a user's role by clicking the pencil next to their name in Users tab.

You can also change their role from the user's profile. In the User Profile tab Permissions section, click Edit.

Either choose a new user role or remove the person's access to the Admin side of Giving.

When someone's access is removed, their activity in the logs will remain intact, along with any other information on their Donor Profile.
Notice
Giving user access is also removed when the person is set to Inactive in Planning Center People, which happens when a person leaves the church, passes away, or any other reason determined by the organization.