To set send as permissions to a group you can use Active Directory Users And Computers. Follow these steps
1. On a computer that is running Exchange, click Start, point to All Programs, point to Administrative Tools, and then click Active Directory Users and Computers.
2. In Active Directory Users and Computers, on the View menu, click Advanced Features.
3. Expand the domain node, and then click Users.
4. In the details pane, right-click the user for which you want to grant the Send As permission, and then click Properties.
5. In <User> Properties, on the Security tab, click Advanced.
6. In Advanced Security Settings for <User>, click Add.
7. In the Enter the object name to select box, type the name of the mailbox user or the group to which you want to grant the Send As permission, and then click Check Name to verify the user or group. Click OK.
8. In Permission Entry for <User>, in the Apply onto list, select This object only.
9. In the Permissions list, locate Send As, and then select the Allow check box.
10. Click OK to close the dialog boxes.