You can grant levels at any level as long as you are higher, you don't lose them after you go above 60. The last level you can grant is level 60, but you get a full level of XP when you grant, so if you do it at 59.9999999 then go explore one area you will be 61.

What will happen in your grant chain is that your last account (the one that didn't refer anybody) will never have grants to receive, however what you can do is level up the 5, then grant down to 4 vet accounts. Granted levels also give more granted levels- this means that if you level the first set to 60 you can then level up another set to 31, grant your 29 to them... you will then have 4 60s with 29 more levels to grant... grant down to the 3 vets linked to those 60s, etc. It's not the holy grail of easy 60s, but it's marginally better than leveling them up and a good option if you aren't looking to make class 5 packs (you could still level 5 packs to 31 while doing this, it's just the ones on the end won't have a 60 with grants to get the lower chained ones up to 60).

There is no limit on total levels you can be granted (although a single character can only grant 29 levels)