Can't add new user gpg keys

Hi,

I've setup git-crypt in one of my team's repos. Got it working for me locally using the `git-crypt add-gpg-user ` the first time.

However, my colleagues can't add their key to the repo and therefore, can't view the encrypted files. I also tried to delete and re-clone my repo and add a new gpg key for myself and all I got was:
`git-crypt: Error: Unable to open key file - have you unlocked/initialized this repository yet?`

These are the steps I took:

* Cloned the repo
* Created a new gpg key with `gpg --full-generate-key`
* cd repo and `git-crypt add-gpg-user `
* The above error is showing

I have a feeling there is something obvious I'm missing. Can anyone help clear this up?

**EDIT** thought it would be worth mentioning, that typing `git-crypt unlock` shows this message:
```
Error: no GPG secret key available to unlock this repository.
To unlock with a shared symmetric key instead, specify the path to the symmetric key as an argument to 'git-crypt unlock'.
```
I thought that using gpg means each person can have their own key?