I’m adding multiple collaborators within a single column separated by a comma. Unfortunately, asana picks the first person and adds them as a collaborator, ignoring the rest of them. I’ve checked the encoding settings, tried adding collaborators within double quotes like this -
“person1@company.co.uk, person2@company.co.uk”
Am I missing something here? 
Hi @Navik and thanks for reaching out! Please note that as it stands you can only add one collaborator per task via the CSV importer; more info in this Guide article. I would also recommend adding your vote to this thread: Add Collaborators during CSV Import 
as it stands, this is insufficient. The importer needs to be able to manage the importation of several collaborators. it wouldn’t be difficult to add this simple feature
@Navik, it appears adding multiple collaborators via CSV import is possible now.
Check out this Asana Guide: CSV Importer
“You can add several of your teammates as Collaborators on a task by adding email addresses of valid Asana users from your workspace or organization on a separate column. Be mindful to separate them with a comma and do not add any spaces in between.”
As far as I can tell it is still recognizing the commas separating the collaborators as separate columns, so it is still not able to import multiple collaborators.
Check that, by doubleing the commas and putting the field in quotes i was able to get it to work.