Hi Geo,
I just exported my contacts to a spreadsheet and the contact group column looks something like this: Lead or Prospect;#Customer;#Vendor
Now, if you add ";#" to the beginning and end of the cell for the contact group column you'll be able to paste back into Contact Manager without any errors. So, the above cell would now look like this: ;#Lead or Prospect;#Customer;#Vendor;#
This seemed to work for me. Let me know if this is what you were looking for.