You don't "add" members to a Dynamic DG, you adjust the recipient filter to include whichever objects you care about. For example, you can use the following to "add" an object with a specific Name value or specific email address:
New-DynamicDistributionGroup -RecipientFilter "Name -eq 'DG' -or PrimarySmtpAddress -eq '******@domain.com'" -Name test -WhatIf