I'm suggesting lots of stuff lately. I feel like it might be too much.

But yeah, here's another one:

For every friend which joins Galaxy Legion via your invite (This can be tracked through FB, I'm sure) you get 10% (Example percentage) of their GP purchases in GP.

The people you've invited don't lose GP, but if they buy, say, 550 GP then you get a complimentary 55 GP.

This promotes incentive to invite people and keep them engaged with the game. It's not an immediate pay-off.

I stole this idea from a chat room which gives benefits to people who donate. If you referred someone to the website, you get credit equivalent to 10% of any donations the people you referred make.

Multi-account abuse is possible with this suggestion. If I buy 160GP on a multi-account which got "referred" by my main account, I now have eleven storages instead of ten.

10% is a bit too high, and way too vunerable to multi abuse, as you stated.

I'm not too sure if "way too vulnerable" it quite how I'd put it. At the end of the day, folks still have to actually purchase that GP in the first place. But yes, it is open to multi-abuse and solutions to that would go along way.

Also, I'm not sure 10% is way too high. Maybe cap it, like "You gain GP equivalent to 10% of your referees' GP purchases up to a maximum of 100GP per month."

Wouldn't it help your ship more to buy GP with your account, then to buy GP with a multi and only get 10%?

Some GP objects, storage towers for example, are tradable.
So IF you wanted to buy tradable objects, buying with a multi will mean 10% free GP on your main plus whatever you are buying anyway through sending.
However, if you wish to purchase non-tradable objects, you will have to directly buy on your main. So it depends on what you wanna do. If you want to buy rifts or modules, for example, this won't help you. But storage, planet flux etc commonly traded GP commodity is open to abuse.

To the suggestion itself: I've always hated spammy game invites, I get a lot of those. I'd be only ok with this if it cost GPs to send invites, which are refunded if and only if the invites are accepted. So if you spam, you lose. You better at least talk to the person you are sending the invite to before you spam them.
On the matter of the multi abuse, at least direct trade from referred to referring player should be blocked(but not the other way around if you wanna give your newbie pal a hand) While it is still possible to do it through a "3rd party", at least it'd be harder.
Or maybe as opposed to say 10% on GP purchased, make it 20% on GP spent on non-tradable(directly or indirectly) purchases, such as modules, energy refill and the like minus the start/daily GP recieved. A bigger % but more restrictive, anyone who is willing to spend will probably at least buy a set of FATHER, so that'd be something in the least. If they are not gonna spend anyway the two methods would not differ.

The game would never prompt you, or at least be unobtrusive, about inviting friends. Even people learning about Galaxy Legion via the "Tell your friends!" thing would count as a referral, assuming they follow the link provided by your FB post to get to Galaxy Legion.

Non-tradeable only doesn't really sound like it would work to me from a "coding" point of view; how to you distinguish between GP you purchased and GP you got for referrals? If you have both types of GP, which one gets spent when you purchase non-tradeables? The answer here is obvious but implementing the code for that is another thing altogether.

I do like the idea of being unable to receive stuff from the account you refer. Maybe this should be for a set period of time OR ranks. E.G. you can't send stuff to your referrer until you reach at least rank 100 (Or some other rank) or until you've been playing for at least 3 months (less optimal because someone can just make a multi and let it sit for 3 months) or even a combination of the two. This does hamper a multi's ability to send stuff to their main account.

Also consider that when Dan is suspicious of multi-abuse, looking at transactions between a referrer and a referee could be of use. For example, player get referred to the game and immediately purchases 550GP and buys, out of that and their starting GP, 10 planet fluxes and 10 arti storages. Then either suddenly all of this crap is sent to their referrer or alternatively once it's all traded for CTP, all that CTP heads off to the referrer. This screams multi to me. This assumes that Dan can track this information.