Hey there TempStar
Thats a great question and thanks for using QandA platform
So the most effective method to share groups of external contacts with a specific department in Outlook is by using shared mailboxes and distribution groups. Firstly, create a shared mailbox dedicated to managing external contacts for the department, granting permissions to the assigned personnel to manage these contacts.
Within this shared mailbox, establish contact groups for each volunteer group to facilitate easy management and distribution of emails. With Exchange hybrid's synchronization capabilities, updates made to contact or distribution groups within the shared mailbox will work across the department, making good access to updated contact information for all members.
If this helps kindly accept the answer thanks much.