The prize for joining with the invite link should be for both sides, so there will be an incentive for those using the link; otherwise, why would they join with the link when they can join without it.
It could be an extra one week of premium for joining with the invite link.
(edit: I'd personally suggest one week of prem. For both the inviter and the invited person)
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*enough atwar, leaving it for the game of real life