Sameroom lets you share your Slack channel with a URL. Whoever you send it to can connect to your channel from their own Slack, HipChat, Skype, Hangouts, Fleep, Flowdock, etc.
To do this, go to https://sameroom.io/create-a-portal, sign in with Slack and select your channel or private group (create it first, if necessary).
Here's a video with step-by-step instructions.
If you run into any issues, we're answering questions on Twitter!