Clearly opt-ins can only be provided by the user herself, rather than nominated or implied as part of someone else's preferences.  With this in mind, what would Best Practice dictate can be done with extra email addresses submited by one customer during a group booking purchase (for example when booking an event for oneself plus several colleagues)? Is there any legal way these customers can be contacted for non-purchase-related reasons, if only to ascertain their marketing preferences?

Kind regards.

Sounds a bit like the rulings on 'viral' email systems or refer-a-friend forms.
It's OK for the end user to nomminate to have an email sent to their friend, not much different than them sending the email themselves.

What's not allowed is for you to store and carry out further processing on their personal data. In short, send it, but don't store their email afterwards.

The DMA or the Data Protection Registrar are usually very helpful on giving guidance in these cases.