You can provide anything that you have permission to use.
re: "Can I use samples of work I've completed for other clients or is that considered private?"
Well... if I did work for a client, that work belongs to the client. I don't really have any rights relating to that work. But I can ASK the client if I may use it in my portfolio and/or share it with other prospective clients.
re: "I haven't signed an NDA but there is a clause in our Upwork agreement that says that we cannot use any of the work for our own purposes."
Signing an NDA doesn't matter. There is similar language built into the Upwork default agreement between clients and freelancers.
re: "I just want to be clear about what we are allowed to use in our portfolio of work."
You can put pretty much ANYTHING into your porfolio, as long as it meets these criteria:
- you made it
- you own it [or] you have permission to put it into your portfolio
