Similar to the previous answer, but different:
you are multiples of $9$.
We are of a family of eleven
When we join or depart numbers less than a cent
$9,18,27,36,45,54,63,72,81,90,99$ - these are the family of eleven numbers you get by multiplying $9$ with numbers less than ten.
Excepting those special ten
We turn odd to even and even to odd
Like a tiny miracle of god
If you multiply $9$ by something with an odd number of digits, you get something with an even number of digits, and vice versa - unless the number is less than $11...1$. Those aren't just ten exceptions though, so probably there's a better explanation for this clue.
However, in general our company results in swap
Of the original ones
Swapping the digits in any of $9,18,27,36,45,54,63,72,81,90,99$ keeps you within the same set.
an easy sum for a clever chap
Summing the digits of any multiple of $9$ gives another multiple of $9$.
I'm not sure about all of the clues, but this seems like it might fit?