Who should be on the invite list to the 'post wedding function'?

Si & Gary 17/10/2017

Si's eldest daughter Samantha is getting married this December!

She's the first daughter to tie the knot, so Si is naturally very excited (and a bit green) about the planning process.

He's heard a few things recently about the family putting on a lunch of some kind the next day - but who should be on the invite this? Is this kind of event usually for friends or families?