I can't see any alternative but to get 4 new accounts. Also your groups will have one member on each account.

Keep account 1 as is then invite new account 2 then 2 gets 3 etc. Now you are all set for exp bonus. Youll be one mount short but that dosnt seem to be your concern.

Current 1 linked to New 2
New 2 linked to New 3
New 3 linked to New 4
New 4 linked to New 5
New 5

Now just trasfer as much as you can afford but make sure each member of each of your groups is on a different account.... Then when you log in 5 man group at a time ALL will get 3X exp if they are in the same level range.

What advantage do you feel you have by having extra characters you cannot play at the same time? Like you'll have a shaman group, if you lose with them you can bring your mage group? Or if a enemy team is winning 5X5 you can bring your team that best counters their characters?