All you need to do is have your Slack administrator go to the Apps Permission page then toggle on Approve Apps access.
After that, they can click "Manage approved apps", which will bring them to the list of integrations added on Slack. If they don't see Tettra, they'll need to install it first by going to https://app.tettra.co and running through the login process.
Your admin just needs to hit the Approve button for Tettra, and then it should be all set.
The final result should look like this. Once it's all set, everyone should be able to log into Tettra.