public class PeopleDTO
{
    public string Name { get; set; }
    public IEnumerable<string> GroupNames { get; set; }
}

var peopleWithGroups = await _dbContext.People.Select(p = > new PeopleDTO
{
    Name = p.Name,
    GroupNames = p.Group.Select(l => l.GroupName).ToArray()
});

return peopleWithGroups;

public class PeopleDTO
{
    public string Name { get; set; }
    public IEnumerable<string> GroupNames { get; set; }
}

var peopleWithGroups = await _dbContext.People.Select(p = > new PeopleDTO
{
    Name = p.Name,
    GroupNames = p.Group.Select(l => l.GroupName).ToArray()
});

return peopleWithGroups;