Apple limits the number of active Distribution Certificates to three, if you're planning to sign apps from more than three machines you'll need to export your certificate and private key to share it.
Distribution Certificates are required to sign the app to confirm it's coming from the correct developer, and Apple limit three active at any time.
Using a Mac with the distribution certificate installed:
- Open Keychain Access (Application -> Utilities)
- Click on My Certificates
- Find the certificate, it'll be named iPhone Distribution: Your Company Name
- Expand the certificate using the Reveal More arrow
- Select both the certificate and the private key
- Right click and choose Export 2 items...
- Specify a name and location for the exported .p12 file
- Enter a password to secure the export
- Share the .p12 file and password (send the password separately if you can)
If you have your own internal app team it's worth considering Fastlane Match as an easy and secure way of sharing certificates through git.