Step1: Connect MSOL Services
Connect-MsolService
Step2: Find out ObjectID of the Security Group you would like add members
Get-MsolGroup –Maxresults 100000 | Where-Object {$_.DisplayName -eq “Test Security Group”}

Get-MsolGroup –ObjectId “af407072-7ae1-4b07-a0ca-6634b7396054”

                             (Or)
Sign-in to Portal.Azure.Com and Select Azure Active Directory -> Security Groups -> Search the Group -> Go to properties of the group and copy the ObjectID 
Step3: Create a CSV file with a header UserPrincipalName and list all email addresses in one column of CSV file e.g.
UserprincipalName
Sharatha@globalspsolutions.com
Test@globalspsolutions.com
Step4: Execute the Below PowerShell Script to add users into the Security group
Copy the script and paste it in a notepad. Rename the notepad to Add-MsolGroupMembers.PS1

################# Start #######################################
Import-CSv -Path “c:\Temp\testscript.CSV” | ForEach {

$UPN=$_.UserPrincipalName

$Users=Get-MsolUser -UserPrincipalName $UPN

$Groupid = Get-MsolGroup -ObjectId “0c3c9f82-2392-43cc-bc00-b0d7b5734ac4”

$Users | ForEach {Add-MsolGroupMember -GroupObjectId $GroupID.ObjectID -GroupMemberObjectId $Users.ObjectID -GroupMemberType User}

}