In your sidebar on the left you can find your current contacts and groups. These are not all your contacts, but the most commonly used ones. Grape sorts your contacts and groups automatically, to give you an overview.
Search all contacts
Browse all available groups and contacts. Hit the search bar ("Search people and groups") all the way on the bottom left and enter any name you are looking for.
The shortcut CTRL + K or COMMAND + K brings your cursor immediately into the search bar (Fast Switch).