Dear Jeff:

No, the Flagellants were never an official group. They were tolerated and even well received by the Pope at the VERY beginning. But, this fanatical group quickly went into heresy.

To positively examine this I would suppose that the original idea was to profoundly share in the suffering of Christ and to mortify the flesh of concupiscence -- both of which the Bible certainly instructs us to do.

But this emergence in the 13th Century was fanatical and extreme, and as often the case with fanaticism quickly became heretical. 

As stated in the Catholic Encyclopedia, "Although as a dangerous heresy the Flagellants are not heard of after the fifteenth century, their practices were revived again and again as a means of quite orthodox public penance."

The Flagellants of today are seen mostly in Latin America where very severe forms of mortifications take place including an annual crucifixion in which a man volunteers to be literally nailed to a cross.  Pope Leo XIII did regulate and restrain some of the practices in Central and South America. I am not sure how the bishops today in the area view those practices.

The advice of the Church today, in general, is that severe mortifications are not to be attempted by the Faithful without the guidance and permission of a wise spiritual director and, depending on the practice, without permission of of ecclesiastical authority.

For more information see the Catholic Encyclopedia article on Flagellants.

God bless,
Bro. Ignatius Mary