janny-jan, you can do this by creating a group in your contact list and then put the group list or recipients in the Bcc field. "Bcc" stands for blind carbon copy. It hides all the addresses in it so they won't see each other's email addresses.
To create a group list or contact category:
1. Click Contact list at the lower left side of the page.
2. From the action bar above, click “Categories.”
3. Choose “New category…” from the drop-down menu.
4. Type a group name in the “Name: box.
5. Type the name of the members in the “Members” box. Auto-complete works on this box.
6. Click “Save” above.
To send an email using Bcc:
1. Create a message.
2. Click the “Show Cc & Bcc” link at the upper right side of the message header. The “Cc” and “Bcc” addressee fields will then appear.
3. Type the group name in the “Bcc” field.