Custom fields is permission based in Rindle. By default, the account owner (the person who created the Rindle account), will have access to custom fields. The account owner and any admin can grant users Custom Field Manager permission when managing their user account.
Add Custom Field Managers
Go to your Account
Then select Users from the navigation menu
Click on the user you want to make a Custom Field Manager, and select Custom Field Manager from the permissions list
Click Update to save the change.
Note: this permission can also be set when creating a new user account.