I often want to share an old email chain with a friend -- usually someone who was on on the chain in the first place.
Is there an easy way to send a "link" or otherwise share that email?
There is no way to get a link or to share an email or thread with another person using just Gmail. Gmail gives you a unique URL in the address bar for each message thread but that URL is only accessible after you have logged in to your account, so another user would not be able to access the link.
Gmail Labs has a Create a Document feature but the document it creates only seems to include the last email in a thread and there is no easy way to copy the remainder of the thread into that document for easy sharing.
One possible solution would be to print the thread as a PDF and send that.
Another possible solution would be to Forward All the thread to an Evernote account and use Evernote's share by URL feature to privately share the note.
There may be other 3rd party web-apps that can achieve this - worth a Google.
You can use GrexIt (http://grexit.com) to share email chains across multiple users. You create a shared label first and then apply this shared label on an email conversation and the conversation will immediately start showing up in other users' inbox. All followup emails also keep getting shared automatically too.
I hope this helps.