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You can use the field on the right to customize the text in the invitation message that will go out to all of the invited users.
If you have created groups in your space using the "Manage Users/Groups" section in Space Admin, you may also select the group that the invited users will become members of after they accept the invitation. To do this click on "EDIT" and check off the groups you want them to be enrolled in. This will allow you to define the permissions that the users will have in your space.
After those steps are completed, click the "Preview Invitation" button. Use this screen to verify that your invitation message is correct. This page will also let you know how many invitations will be sent. If you need to go back and modify something, simply press the "Re-edit" button. If everything looks like it is ready to send, click the "Send" button.
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