Thanks for the report! I've confirmed that this is a bug in the current BETA release:
1. Contact email addresses should auto-populate into the To: field when you click Send Email on one or multiple contacts.
2. Categories are intended to help organize and filter your contacts, notes, tasks, and calendar appointments. For emailing groups of addresses, we actually recommend creating an email alias. An email alias is essentially a forwarding email address that can be used to forward messages to a single address or multiple email addresses. After you create this alias, you would just enter the alias name in the To field of your new message.
That said, when #1 is fixed up, it should be simple to also mass email the contacts in one of your categories. For example, you would go into your Contacts and hide the Global Address List view. Then use the Show/Hide buttons next to the categories to view only the category you'd like to send to. Click Select > Select All and then click on the Selected button once more to choose Send Email from the dropdown list. When #1 is fixed, all of their email addresses will auto populate in the To field.
I hope this helps!