private class Group
        {
            public string Name { get; set; }
        }

        private List<Group> _groups;

        private string Check(string newName, Group group = null)
        {
            while (_groups.Where(gr => gr != group).Select(gr => gr.Name).Contains(newName))
            {
                newName += "1";
            }

            return newName;
        }

        public void DoAdd()
        {
            var group = new Group();

            group.Name = Check(group.Name);
        }

        public void DoRename()
        {
            var group = new Group();

            group.Name = Check(group.Name, group);
        }