I thought this idea had been turned down already, but nothing came up from a quick search. So I guess I suggest this as a brand new idea, or if my search missed it, a new twist on an old idea !
I think few would oppose the idea that a giving a friend a pre-activated sim, with a little credit on it, would be an easier way to get them to try giffgaff, and hopefully get on board.
I propose a way of doing this that would not cost giffgaff anything beyond a (possibly) small initial cost just to set the idea running. We, the users, would bear most of the cost initially, but with luck would see that cost turn into a profit.
As well as the current method of ordering un-activated sims there could be the option of using our credit to buy sims already activated with, say, £2 credit. For this we would pay £3, and the extra £1 would offset any cost to giffgaff.
Even though the credit on the card would be paid for by us the card itself would remain effectively ownerless until the recipient (hopefully) sees how excellent giffgaff is and goes through a registration process similar to activation to become a full user. Once that happens we get the usual 500 point reward, which turns our small loss into a profit, but the new user does not get the welcome £5.
Additional safeguards against abuse would be that the sim is only valid for the usual 6 months unless in use, and that the sim must be registered within 30 days of first use.
Obviously we would have to be a little more selective about who we hand these sims to if we are not to stand too many losses, but I think it would reduce waste, and the take up rate would be higher.