I have the SAME ISSUE! Help!
For any SingleSelect or MultipleSelect field, in this case that’s your {Platform}
field, you can add a new option by clicking the down arrow next to the field name and clicking “Customize field type”. Or, in any record just start typing something like ‘LinkedIn’ in that column and you should let you add a new option.
For your grouped view, you may find it useful to turn on ‘Show groups with no records’. You do that by going into your Group settings and clicking the eyeball icon to the right of that group.

I have the SAME ISSUE! Help!
I had to quit overnight and come back to it to figure it out but I DID!
Go to the Platform Column and right-click on it or click the drop-down button.
Customize Field Type. Add in whatever social media platforms you need.
Go to the plus sign under the last entry (say, your last Facebook entry at the top). Change the Platform drop-down name to Twitter or YouTube, etc. and it will automatically create a new box for you! You can also click the arrow to the left of the platform name and it will collapse everything so you can see a long or short list, depending on your preference.
I hope that helps!!
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