To share a memory, you have to put it into a shared album. First, create the album in the Albums menu, then share it with your loved ones.
To share an existing memory, go into the Memories menu, open the memory you want to share, click on the Add to albums icon, and choose to which album you want to add it.
If you are adding a new memory, before saving it, click on the Add to albums button and choose the shared album you want to add it to.