Lightsail comes with an admin account but you don’t really want to give this to third parties if they require access for support. This is how to create (and remove) additional accounts.
First, use PuTTYGen to create key pair. The private key will be a .ppk file and the public key will be copied from the text area of the GUI into a .pub file. Save these files in a safe location.
Copy the .pub file to bitnami home directory on host.
Use Step 2 (Option A), substitute USERNAME for the account to be created.
sudo useradd -s /bin/bash -o -u `id -u` -g `id -g` USERNAME
sudo mkdir ~USERNAME/
sudo cp -rp ~bitnami/.ssh ~USERNAME/
sudo cp -rp ~bitnami/.bashrc ~USERNAME/
sudo cp -rp ~bitnami/.profile ~USERNAME/
cat USERNAME.pub >> /home/USERNAME/.ssh/authorized_keys
If they need administrator access
sudo usermod -G bitnami-admins USERNAME
Then follow instructions for log in using either ssh or SFTP.
To remove user account
sudo userdel USERNAME -f