If there is someone using an email exchange that isn't Google you will have to get them to create a Google account to be able to edit files in Google Drive.
This is how you do it.
- Go to the Google Account Sign In page.
- Click Create account.
- Enter your details.
- Click Use my current email address instead.
- Enter your current email address.
- Click Next.
- Verify your email address with the code sent to your existing email.
- Click Verify.
This will create a Google account which should let you use the full features of Google Drive.