I think that there must be a refer codes in game which may give benifit of units.
Location - Near 'sign in' box,in settings.
Benefit - 20× units to the existing player (whose unique code will be used)
And 40× units to the summuner who uses it.
Eligibility* - Account must be at least 1 month old to use code and to share account must be atlest 5 months old.
Restrictions* - Each summuner's code can be used for 10 times only on different accounts. And one summuner can add code for only once.
*Restrictions are important otherwise youtubers will farm millions of units.
*It is also imporant to set elegiblity otherwise everyone will make 10 accounts of their own and will use code and will leave playing it.