You can add existing animal records to a litter in one of two ways.
A. From the litter page
On the litter page, the "Add to this litter" button now has two options to choose from - "Add existing" and "Intake new."
If you choose the first option, you'll land on a new page where you can select the animal(s) to add to the litter. The filters on this page will default to search for the same species of the existing littermates. You can always change this filter if needed.
Click the green "Add Now" button for your desired animal(s), and they will be immediately added to the litter.
NOTE: Only animals that are currently in your custody can be added to a litter.
B. From the animal's Medical tab
Another way you can add existing animals to a litter is by going to the record of one of the animals already in the litter. On the Medical tab, you'll see an "Add to this litter" button with the same two options, "Add existing" and "Intake new."
If you choose the "Add existing" option, you'll be taken to the same page to select animals as described above.
NOTE: You will only see the "Add to this litter" button on animal records that are already part of a litter. For animals that are not yet part of a litter, you'll see a button for "Create a litter."