How do you register as a Hindu undivided family?

There is no need for formal registration for an HUF. If you are eligible to form an HUF you can simply open a bank account , obtain a PAN number and start business in the name of the HUF. There is no need for any agreement between the members as such. However to open the account with a bank and for some other formalities a delcatation has to be made delcaring the HUF and the member so the same.