unable to allocate users to multiple groups during bulk import

[daj1u06]

Librebooking 2.8.6.1.
Using the bulk import users option and I want to allocate users to multiple groups.
When you export the users, if a user belongs to multiple groups, it comma separates the groups in the exported csv, which is obviously no good if trying to import. I've tried quotation marks and semi colons as separators within the field and update user optionis selected but to no avail, nothing goes in.
If I create multiple lines in the csv, one for each group the user belongs to and use the update users option, only the last line goes in.

What is the correct syntax for doing this, or is it a bug in the importer?

Thanks in advance.