Hi Sriram,

You can write a trigger on User object and use below code to assign permission set

        String  PermissionSetName = 'ReadAll';
        List<id> userIds = new List<id>();  // list of id's from the trigger
        
        List<PermissionSetAssignment> permissionSetList = new List<PermissionSetAssignment>();
        PermissionSet readAllpermission = [SELECT Id  ,name FROM PermissionSet where name = :PermissionSetName];
        System.debug('readAllpermission:::'+readAllpermission);
        
        List<string> existingRealAllUserList = new List<string>();
        List<PermissionSetAssignment> FunnelReadAll = [Select AssigneeId FROM PermissionSetAssignment WHERE  PermissionSet.Name = :PermissionSetName];
        
        
        for(PermissionSetAssignment p: FunnelReadAll){
            existingRealAllUserList.add(p.AssigneeId);
         
        }
        System.debug('existingRealAllUserList:::'+existingRealAllUserList);
    
        List<User> userList =[Select id   from User Where IsActive = true AND id IN :userIds];
        for(User s : userList){
            
         if(!existingRealAllUserList.contains(s.id)){
             
            PermissionSetAssignment psa = new PermissionSetAssignment (PermissionSetId = readAllpermission.id, AssigneeId = s.id);
            permissionSetList.add(psa); 
         }
         
        }
    
        try{
        if(permissionSetList != null)
            upsert permissionSetList;
        }catch(exception e){
            system.debug('exception caught' + e);
        }