Make sure that the defaults editable box is checked in the lobby ambassador account - then the time limits can be altered by the lobby ambassador when editing the guest accounts (under the advanced tab).
You can set a value of 0 on the lifetime of the new user. This then removes the user from the Lobby Ambassador list, and creates a permanent user account. The user is listed under the main Admin account in - Security > Local Net Users, and can be deleted when no longer required.
I have the same issue. A customer uses the lobby ambassador account to create guest accounts. When they create a guest with lifetime 0, this user dissapears from their list of guest users. The customer wants to see this user also, and want to delete it if neccessary.
A way around is then to set the lifetime as much as possible, but 30 days max is not long enough. Is there another way to solve this issue?