Sharing keys on Ubuntu is as easy as:
If you want to use a specific key, then use the following
ssh-copy-id -i [PUBLIC KEY FILE] firstname.lastname@example.org
If you don't already have an identity, an error will pop up and you will need to run the following before retrying.
Sometimes you need to do things manually, such as when you can only use a key to log into the server and need to add another key. In such situations, just copy the public key (which should look something like below)
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCpPhKCzJUS6RGRza+FudimGKR2RILtShMyN1Gsd0V7r3H1vJ2WwwgMGjUQmow6IyFEFH+1TlbCGDTGxRA82G5+vW3NCbbtkTXyEJsVQ7/QkLqnQL4++BRLhJ5UAzKv84Ohw45VeJPVtW/gAqvB7r4+vcRNPrW3dRW8TVgF9fAi5RdacDXBBGFkSE1IA5mAMqDmiXryn4SlqFqeS4n/jsEFsCmOPo1S67xqp/QsVYyZHjS9jFwxxLuSj/7VT0QuQcU1Q7QWifGM4Rdj8jIboxEoM59Ws9GpeJK94UUX+L8vg8QUp+wiEdQnrzyaTVbu0NtPUdsmynNcM06XWM74E2bT stuart@stu-home-office
... and append it into the
$HOME/.ssh/authorized_keys of the user that you want to use the key to login as. Note that this is a single line that will look like an entire paragraph with the wrapping. The bit at the end such as
stuart@stu-home-office is just an identifier that you can change to be something more memorable. E.g. to let you know what other servers/users are granted access to the server so that you can remove them at a later date and not accidentally remove the wrong key.