Praise be to Allah.
With regard to offering expiation, it is permissible to invite the required number of poor persons and make food for them sufficient for them to eat their fill, of the average kind of food that the one who is offering expiation usually eats.
Ibn Abi Shaybah narrated in al-Musannaf (7/533) from Anas (may Allah be pleased with him) that he fell sick before he died, and he could not fast, so he used to gather thirty poor persons and feed them bread and meat in one gathering.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said:
If he makes food and they eat lunch or dinner, then he has fed them… Hence Shaykh al-Islam (may Allah have mercy on him) said: So long as the Lawgiver did not define the amount or time of food to be given to the poor, then whatever is called feeding people is acceptable, even if it is offered as lunch or dinner.
End quote from ash-Sharh al-Mumti‘ (15/160-162)
With regard to inviting people to the wedding feast with the intention of offering expiation, that is not acceptable, because by doing that one is protecting one’s wealth (and saving money). Moreover, the expiation of feeding sixty poor persons is an expiation that is aimed at disciplining and rebuking, so making it as part of the wedding feast is not appropriate with that aim and does not fulfil the purpose of offering expiation, unless one goes to the expense and trouble of making special food for that and specifically invites sixty poor persons in addition to the wedding guests, so that he invites rich people for the wedding feast and invites poor people with the intention of offering the expiation.
We put this question to our shaykh, ‘Abd ar-Rahmaan al-Barraak (may Allah preserve him) and he said: He does not have the right to do that, unless he pays extra costs and makes an extra effort for them and adds special food for the purpose of offering expiation. End quote.
See the answer to question no. 153980
And Allah knows best.