Alias

HelloRoger Roger ! 

There are several ways to get this done. 
I'll give you one GUI based method and one PowerShell /Exchange Shell method 

GUI Version: 

1. Open EMC ( Exchange Management Console ) on your Exchange server
2. Go to "Recipient Configuration-->Distribution Group" 
   
3. Right click your distribution group and choose "properties" 
4. Choose the "E-mail addresses" tab 
5. Click add
   
   
   
   

Shell method:

1. Open up EMS ( Exchange Management Shell ) 
2. Type the following commands note that my groups name is sales, replace sales with your own group. Also replace "alias@domain.com" with your wished alias.

$Group = Get-DistributionGroup "sales"

$Group.EmailAddresses +="Alias@domain.com" 

Set-DistributionGroup -Identity "sales" -EmailAddresses $Group.EmailAddresses

The 3 above commands will do the following

1: Gets the Distribution group and saves it to a variable called $Group

2: It gets the current email addresses of the group and then adds "alias@domain.com" to that list in the variable

3: Sets the groups email addresses using the list from the $Group variable. 

Let me know if this helps!
Feel free to mark my reply as a solution if it did

If you have more questions, let me know! 

Kind Regards
Oliwer Sjöberg