Users are automatically synced between your Slack workspace and Cupcake. Your entire workspace (excluding single channel and multi-channel guest users) will be available on Cupcake. The only way to add or remove a user is to add or delete them to your Slack workspace.
However, a user must be apart of a group to celebrate their birthday. See "What are groups and how do I use them?" to learn more about using groups to celebrate birthdays.