Quite often I will have conducted the cremation ceremony and then the family will ask me to intern the ashes at a later date. This can either be into an existing grave, a brand new one or it may even be an ash scattering. Scatterings can pretty much take place anywhere so long as there are no restrictions, or you may choose a natural burial ground. Crematoriums also have dedicated places to scatter ashes.
Who you invite to the internment/scattering ceremony is entirely up to you but again I would write the ceremony for you and email a copy to you for your approval. These ceremonies are not normally very long, and can include music, prayers, poems, readings.