...
To do this while logged in to the instance, you can simply append the public key to the authorized_keys file directly using copy and paste (e.g. nano ~/.ssh/authorized_keys then paste in the string of characters from the public key). If the public key has been transferred to your instance, the contents of the file may be appended using the following example:
...
Replace ubuntu with centos in the examples above if your instance was created using one of the CentOS images.
So to summarise, there are 3 levels of Nimbus access:
1. Project owner + Member, can do everything, is responsible for everything
2. Member, can do everything (given Project Access)
3. Instance user, granted access by Member after instance created