Heidi's answer is the right one.

Do you mean we should say ‘when you’re invited for 7, you’re expected to come by 6:30

'invited at 7' = at 7:00: "Would you like to come?" so the plan can't be for 7!! You mean 'invited for 7' and expected to be there 'by 6:30'

You can also say 'If you're invited at 7, you're expected to arrive by 6.30.'