How to get the group members user details (Like user id, username) from Data loader?

• Create an Apex class.
  

• The Apex class name is getUserListClass.
  

• Copy and paste the following.

public class getUserListClass {
    
    public static List<User> getUserList(String GroupId){
        List<GroupMember> GroupMemberList1 = [SELECT Id, GroupId, UserOrGroupId, Group.Name FROM GroupMember WHERE Group.Id =: GroupId ];
        
        Set<Id> UserIdSet = new Set<Id>();
        Set<Id> GroupIdSet1 = new Set<Id>();
        Set<Id> GroupIdSet2 = new Set<Id>();
        Set<Id> GroupIdSet3 = new Set<Id>();
        
        for (GroupMember GM1 : GroupMemberList1) {
            String id_str = (String)GM1.UserOrGroupId;
            if (id_str.left(3) =='005') {
                UserIdSet.add(GM1.UserOrGroupId);
            } else {
                GroupIdSet1.add(GM1.UserOrGroupId);
            }
        }
        
        if ( GroupIdSet1.size() >0) {
            
            List<GroupMember> GroupMemberList2 = [SELECT Id, GroupId, UserOrGroupId, Group.Name FROM GroupMember WHERE Group.Id =: GroupIdSet1];
            for (GroupMember GM2 : GroupMemberList2) {
                String id_str = (String)GM2.UserOrGroupId;
                if (id_str.left(3) =='005') {
                    UserIdSet.add(GM2.UserOrGroupId);
                } else {
                    GroupIdSet2.add(GM2.UserOrGroupId);
                }
            }
            
            if ( GroupIdSet2.size() >0) {
                List<GroupMember> GroupMemberList3 = [SELECT Id, GroupId, UserOrGroupId, Group.Name FROM GroupMember WHERE Group.Id =: GroupIdSet2];
                for (GroupMember GM3 : GroupMemberList3) {
                    String id_str = (String)GM3.UserOrGroupId;
                    if (id_str.left(3) =='005') {
                        UserIdSet.add(GM3.UserOrGroupId);
                    } else {
                        GroupIdSet3.add(GM3.UserOrGroupId);
                    }
                }
            }
            
        }
        
        List<User> UserList = new List<User>();
        if( UserIdSet.size() >0) {
            UserList = [SELECT Id FROM User where Id =: UserIdSet];
        }
        
        return UserList;
        
    }
}

• Open excute anonymous window

Use 18 digit Id

List<User> UserList = getUserListClass.getUserList('00G2x000000uOKDEA2');
IF(UserList.size() >0 ){
    for (User u : UserList){
        system.debug(Logginglevel.INFO,'-- List -->  '+ u.Id );
    }
}

• Final result