There is limitation on the number of items that make up group.
Is there another function that may better serve what you're trying to do? For example, memorising the estimate and then go to Memorised Estimate list. It will appear there without a customer so you can choose a different customer for the next estimate.
As Rav said you can memorise an estimate (quote) once you select the group item on the quote you can insert many more items within the group then memorise, I presume you do not wish to show the details within the group but rather a summary when printed?