There are two ways an owner or admin can add users to an existing distribution list from the Contacts page.
Option 1: Invite new contacts and add them to an existing distribution list
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Click the Invite contacts button.
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The “Invite New Contacts” window opens.
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Enter the email addresses of new contacts you want to invite to your account.
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Select the distribution list(s) to which the new contacts should be added.
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Click the Send invitations button.
Note:
If you have “All Contacts” selected and a distribution list is not selected when you click the Invite contacts button, the checkbox next to any distribution list will not be checked in the “Invite New Contacts” window. 
If you do have a distribution list selected when you click the Invite contacts button, the checkbox next to the distribution list you selected will be checked by default in the “Invite New Contacts” window. 
Option 2: Using the Bulk Actions bar at the bottom
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Select the checkbox of users you want in the new distribution list.
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Click the Add to existing list button.
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The “Add to Existing List” window opens and includes the list of users.
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Select the distribution list(s) to which the users should be added.
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Click the Add to List button.
