What is a group of tortoises called?
Creep is correct. I sent an email asking this question to The Tortoise Group, which is a non-profit organization whose mission statement is:

Improving the lives of wild and desert tortoises through education.

The Executive Director replied:

It's a bale for turtles and a creep for tortoises. I am sure they could have come up with a better name!

If you have additional questions, there's a wealth of scientific information on their website.