Permissions dictate what a user can do in One Church Software. Grant permissions to each individual person (including whether they have access to administrative portions) from their person profile.
To manage these permissions, you would go to the person's profile and click on "Actions > Manage permissions" in the top right. Note that this option will only appear if they have access to the system already.
You can also get to the same place by going to "Cog Menu > Security > Users Tab" from the top navigation menu. Then, select the "Manage" button next to the person you want to edit. The "Edit User Access Rights" pop up appears.