Sender Users link an individual Salesforce User to a Sender to allow you to control which numbers a User can utilise. This is helpful if you have different numbers for different brands, or want to restrict a number from certain users for certain functionality, for example, allowing only Marketing Team members to use a number dedicated to marketing messages.
From a Sender record, click "New" on the "Sender Users" related list. You should then use the lookup field to select the Salesforce User you wish to grant access to this number. Each user can also have one default Sender number per channel. Checking the "Default" checkbox designates this Sender as the default for this user for any channels the Sender is enabled for. Note that if you have previously selected another Sender as the default for any of these channels, that defaulting will be removed and only the newly created Sender User will be marked as the default. You can see how to create a Sender User in the following video.
