Perhaps there's a limit to how many members Add-ADGroupMember can handle at once, and it croaks if you pass the -Members parameter a collection of hundreds? Instead of grouping all the users into a collection, try adding them one at a time:
Import-Csv "C:\Scripts\Import Bulk Users into bulk groups\bulkgroups3.csv" | %{
Add-ADGroupMember -Identity $_.Group -Members $_.Accountname
}
This assumes that the values in the Group and Accountname columns are valid identity properties--which they'd have to be in order for the script you quoted to work. Note, though, that this script will take a long time to run, because the AD cmdlets usually execute slowly, and you'll be executing Add-ADGroupMember once per user (i.e., per line in the CSV file) rather than once per group. You might find it useful to add a line to the Foreach-Object block indicating progress so that you know it's working rather than getting stuck.