I am managing a companies Apple Developer Account. So there is one app developer who wants to publish his app. He says he needs a distribution certificate for that. He wants it as a .p12 file. The question I have is that the .p12 file will also contain my private key. Isn't it dangerous to forward the .p12 file to someone else, cause I have red that one can extract the private key out of the file again...
The p12 file is a certificate/key pair and will include the private key that you used when generating the certificate. If you provide this to the user then effectively you are giving them enough access to sign the app with the certificate as if it was you. The odds are that you don't use that private key else where so the risk is "probably" restricted to that.
Having said that I believe it's not an uncommon practice to provide a p12 file to developers to allow them to generate and sign an app for publishing.
There are a couple of other options that you could consider. Generate a second distribution certificate with a different key. From memory apple allows up to three of these.
Have the developer sent a build to you which you can resign prior to uploading.
You also might want to check out the related question Any concern to share private key for distribution certificate among different group under a team account in itune provisioning portal