From the Pakistan Constiution
20 Freedom to profess religion and to manage religious institutions.
Subject to law, public order and morality:-
(a)
every citizen shall have the right to profess, practice and propagate his religion; and
(b) every religious denomination and every sect thereof shall have the right to establish, maintain and manage its religious institutions
http://www.pakistani.org/pakistan/constitution/part2.ch1.html
However their is an Ordinance passed by Zia which stated
Any person of the Qadiani group or the Lahori group (who call themselves 'Ahmadis' or by any other name), who directly or indirectly, poses himself as a Muslim, or calls, or refers to, his faith as Islam, or
preaches or propagates his faith, or invites others to accept his faith, by words, either spoken or written, or by visible representations, or in any manner whatsoever outrages the religious feelings of Muslims
shall be punished with imprisonment of either description for a term which may extend to three years and shall also be liable to fine.
So with the exception of Ahmadis every group can preach their religion. Ahmadis are the most discriminated group in Pakistan.