EXCHANGE COMMAND
Import-CSV "File Name.csv" | ForEach {Add-DistributionGroupMember -Identity "Group Name" -Member $_.SamAccountName}
ACTIVE DIRECTORY COMMAND
Import-Csv "File Name.csv" | % {Add-ADGroupMember -Identity "Group Name" - Member $_.SamAccountName}
In both cases you must have a CSV file with a header of SamAccountName and then the user accounts below that. For Example:
SamAccountName
bsm
chh
des
Also, if you use the Active Directory Command inside an Exchange Management Shell, be sure to import the AD module:
import-module activedirectory
You can put in another header if you like (UPN, SMTP Address, etc), but I've found SamAccountName works best for me.