1. Edit 1 of your accounts in all group bills so they have a 0 group balance
2. Remove that account from the group
3. Re-try the merge
More detailed steps:
To edit 1 of your accounts off all bills:
1. Tap expense
2. Tap "Edit"
3. In the sentence "Paid by X and split equally" tap "equally"
4. Tap the name of 1 of your accounts so they become unchecked. They are now removed from the expense
Repeat for all groups bills. Be sure to remove the same account each time. You may want to name one account REMOVE ME or something similar to make this easier.
To remove this account from the group:
1. Open group
2. Tap settings icon
3. Tap grey X next to 0 balance for the account you'd like to remove
To re-try the merge, visit this page: https://secure.splitwise.com/users/merge