You can add extra fields to your collaborators table by creating 'Collaborators Fields'. Fields can be various types - text, textarea, email, date, checkbox, select.
Select type gives you opportunity to create different types of selects. Example:
To create select box, in the select type field choose 'select'. Then you can choose to make it multiple select or not.
You can also create radio button, for that, you should choose 'Radio' in the select type field. If you choose multiple, it will create select checkboxes with the option to choose multiple. If you do not select multiple, it will create radio bittons with only one available option to choose.
To specify options for selects, please press on the 'Add new' button.
You can also choose to make the field required.
As the result, you will see your extra fields in the separate box called "Extra".