First create universal group and add users to the group the mail enable it

With the following command from the exchange PowerShell

Enable-DistributionGroup -Identity “groupName”

After its enabled

Run the following command

send on behalf off

(run the command from the exchange powershell)

Set-MailPublicFolder “\publicfoldername” -GrantSendOnBehalfTo “Groupname”

send as

For the send as go to properties of the distribution  group from the ECP schange link then go to delivery options

Add the group