I have tried to create a group under ArcGIS.com like a million times in vain! This is the message i have been getting in all attempts: 'getElementsByTagName(...).0.value' is null or not an object undefined
This looks like the error you get if you try to reuse a group name you've already used to create a group. Your group names need to be unique (to you). There could also be an issue with the characters you've used.
Verify that you don't already have a group with the same name.
If this group name is unique, can you reply here with the name so we can see if there are any problematic characters?
By the way, we are fixing the error message so it provides more useful information. Thanks Molly
I think I solved my on issue. I was attempting to copy a paste more than a few lines of text into the "summary" text box; when I shortened the "blurb" the application accepted it. It seems that ArcGIS Online likes only 2 lines of text in the "summary" section of groups.