public class Person
{
    public string name;
    public List<Group> groups;
}

public class Group
{
    public string name;
    public List<Person> members;
}

public class PersonDTO
{
    public string name;
    public List<GroupDTO> groups;
}

public class GroupDTO
{
    public string name;
    public List<PersonDTO> members;
}

CreateMap<Person, PersonDTO>()
    .ForMember(x => x.groups.Select(y => y.members), opt => opt.Ignore())
    .ReverseMap();

CreateMap<Group, GroupDTO>()
    .ForMember(x => x.members.Select(y => y.groups), opt => opt.Ignore())
    .ReverseMap();