Figured it out. The value of $myigroup.Initiators is an array. I needed to change it to a string in order to use it as "one value".
To do that I used the string join method:
>([string]::join(",",($myigroup.Initiators)))
Which resulted in:
21:01:00:b0:83:83:de:c5,21:01:00:b0:83:83:fg:c9
Which I could then use as a value in my Export-CSV file.