The members of this organization are not so hidden and not so secretive. This is only a problem of being able to understand who's who. And mostly, where are they operating - and yes, Tunisia is on the list too.
A couple of links contributing to a wider picture:
Moderates, not exactly...
Claire Berlinski, on their origins
Carnegie Papers: The Egyptian Muslim Brotherhood
The Muslim Brotherhood and the last elections in Jordan
The official website, in English - interesting to follow various reactions on the events taking place in the North of Africa and in the Middle East