How to increase PV's in a Volume group ?

I have a Volume group on one of my production box. The no of PV's within the same is 16 which I need to increase.
How can this be done ?
Can this be done online or do we need a downtime for this ?

Please provide the exact details and steps for this activity urgently, as need to do this on my production box.

Please revert if you need any more details. Thanking you in advance.

which os?

if aix 4.3+ it can be done without downtime with

extendvg <vgname> <hdiskname>

It is HP-UX .

You are better off posting HP-UX specific questions in the HP-UX forum. What you need to do depends on several things starting with which release of HP-UX. See this document. I'm guessing that your maxvgs is at 16 and needs to be raised. This means tuning the kernel. The maxvgs man page indicates that a reboot is required.

Reconfiguring the Kernel (Prior to HP-UX 11i Version 2)
Reconfiguring the Kernel (HP-UX 11i Version 2)