genetives (apostrophe)

I teach English and am using your site as a learning tool for my students.

I helped a student with the apostrophe quiz and we got 3 wrong!!
I bought them at the grocer's. ( I thought the apostrophe should be there because it means the "grocer's shop". Without the apostrophe I would think it means that there are numerous grocers.

Did you invite the Browns?
I didn't think we should use an apostrophe there because it only means "more than one Brown family member".

Am I wrong about both these questions?

[Tdol]

You're right about both. Thanks- mistakes do occur because of human error, and we are only too human. However, the weird thing is that the database had the correct answer for the Browns, which I can't explain. Amyway, I have sorted out the other one and am very grateful indeed.