Please help with Apple Photos App on iMac

Please help with our Apple Photos. We recently replaced our family iMac. The old one had user accounts for me and my wife and our three kids. Our photos library was kept on a remote hard that we swapped once or twice a year for safe keeping.

The kids are away at school and my wife and I bought a new machine and decided not to use user accounts. The kids don't use desktops anyhow. I also successfully copied the photo library onto the new machine. Initially it contained all of our "shared albums", but now it does not.

Before when we were invited to a shared album and we accepted on our iPhones, the shared album showed up n all of our devices, including the iMac. Today, if I am invited to a shared album, nothing shows up on the iMac.

About our family: Originally we had one Apple ID. It was from the early days, before kids, before we had iPhones or separate apple identities. It is the ID that has the credit card linked to our account. Later, my wife and I, and eventually the kids, all got unique .iCloud IDs. This may not be what Apple intends with Family Sharing, but it has worked none-the-less. My devices have my unique Apple ID and the original Apple ID with the credit card listed for payments. I don't know if this is having an effect. It would seem that the new desktop, with only the original Apple ID, is now a unique identity.

So I'm wondering if there is a way to get our/my shared albums back? Some way my wife and I can have out unique devices but still use a common desktop? If not, what do I need to change to make the new machine one my personal devices so we can still avoid user groups?

Thank you in advance.