I've been told that community / open-source version of pulumi is only single user... Can someone explain that?

The open-source tool can be used by any number of users. The community tier of the service which provides state storage, history, etc. is limited to a single user - https://www.pulumi.com/pricing/#community-edition. Note: you can use the open-source tool without the service entirely, you just need to take care to store the state file yourself. https://www.pulumi.com/docs/troubleshooting/faq/#can-i-use-pulumi-without-depending-on-pulumi-com

@gentle-diamond-70147 is there an overview of what would be missing if we didn't use the service but just the open source tool? Can we still use encrypted settings for example?

I don't think we have a page that lists it in that manner, no. Encrypted secrets are available in open-source - using a passphrase or one of the external encryption providers. https://www.pulumi.com/docs/intro/concepts/config/#initializing-a-stack-with-alternative-encryption

Perfect, thanks.