[IMP 2.2] Each Contacts List entry can only contain a single email address.
[IMP 3.x] Full support for multiple addresses is still being implemented. But, in most cases you can store multiple addresses in a single Address book entry simply by listing all the email addresses in the Address book Email field separated by commas. Do not use semicolons (";") to separate lists of email addresses.
Note that the number of addresses that fit in a single entry may be limited.