Add User in public group through Apex Code

Requirement # 

Automatically want to add a user in public group.

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trigger addintoPublicgroup on User (after insert) {
   AddUser.AddToGroups(trigger.newMap.keySet());

}

public class AddUser{

@future
public static void AddToGroups(Set<Id> userIds)
{
 //Get the groups that the user should be added to
Group g=[select Id from Group Where DeveloperName='My_Training_App'];

 List<User> users=[Select Id,Name from user Where Id IN :userIds];

 List<GroupMember>listGroupMember =new List<GroupMember>(); 
 // loop the users that have been created
 for (User user : users){
      GroupMember gm= new GroupMember();
      gm.GroupId=g.id;
      gm.UserOrGroupId = user.id;
      listGroupMember.add(gm);  
 }
 insert listGroupMember;
}
}