https://www.alz.org/alzheimers-demen...-loss-concerns is a good basic overview of dealing with early memory loss issues, from the point of view of a loved one OR the person experiencing the memory loss who is not "quite" sure they have a problem yet.
Finding Dementia Care and Local Services offers help finding local support groups and other services to help patients and their families.
If their memory loss is severe enough that you feel they need an intervention, but they're in denial and don't think they have a problem at all (yet) - you could trick them. Just remind them they have the new doctor for their annual physical this year, and take them there. Meanwhile it'll be a doctor who specializes in memory care, who you've already spoken with at length to set up this intervention.