NOTE: Some UI elements in the Dataset Management Experience have been updated since 12/7
In order to add a new column to an existing dataset, you will first need to create a revision of the dataset by clicking on the Edit button using the action bar from either the Primer Page or the Data Table:
Select Review Data to open the data preview and configuration modal:
You can then add new columns to your dataset by clicking on the "Add Column" button on the left-hand side of the data table preview. In this modal, you can choose the Display Name, Field Name, Description, Source Column, and Data Type of the new column.
NOTE: Valid field name characters are all lower case, alphanumeric and underscore. It must start with a letter or underscore and cannot contain special characters or spaces.
Column names are limited to 255 characters (measured in code points). If your column name is longer than that an error will be shown.
Click the Add Column button in the lower-right corner of the 'Add column' section, then navigate back to the data preview by selecting Done at the bottom right.