Can i use SPMT mapping 1 Group to more user?

Hi @Kun-J.S , welcome to Q&A forum!

Under normal circumstances, we do not use SPMT to add users to the group. You can add users on the UI, or you can add users through PowerShell.

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You could:

1. Log in to the SharePoint Online site which you wish to add members, as an administrator.

On the site’s home page, click the Settings gear icon >> Choose Site permissions >> and then click on “Advanced Permissions Settings”. Alternatively, You can click on Settings >> Site Settings >> “Users and Permissions” link under “People and Groups” from anywhere else on the site.

2.Using Powershell:

#Connection paramaters  
$AdminURL = "https://Crescent-admin.sharepoint.com/"  
$AdminName = "******@Crescent.com"  
$CSVFile="C:\UsersToAdd.csv"  
   
#User Names Password to connect  
$Password = Read-host -assecurestring "Enter Password for $AdminName"  
$Credential = new-object -typename System.Management.Automation.PSCredential -argumentlist $AdminName, $Password  
   
#Connect to SharePoint Online  
Connect-SPOService -url $AdminURL -credential $Credential  
   
#add multiple users to sharepoint online group powershell - Import Users from CSV  
Import-Csv $CSVFile | ForEach-Object { Add-SPOUser -Site $_.SiteURL -Group $_.GroupName -LoginName $_.UserAccount}  

Please refer to this blog for more information about how to add users to SharePoint groups:
SharePoint Online: PowerShell to Add User to Group

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