Here you go with this code snippet
public void GetSPUsersOfGroup(string webURL, int groupId)
{
List<spuser> listSPUsers = new List<spuser>();
Web currentWeb = ClientContext.Site.OpenWeb(webURL);
UserCollection userCollection = currentWeb.SiteGroups.GetById(groupId).Users;
ClientContext.Load(userCollection);
ClientContext.ExecuteQuery();
foreach (User user in userCollection)
{
SPUser spUser = new SPUser();
spUser.DisplayName = user.Title;
spUser.Id = user.Id;
spUser.Email = user.Email;
spUser.AccountName = user.LoginName;
listSPUsers.Add(spUser);
}
}