Hey @simkin , happy to help!
Few things to note, make sure you are using a JWT App credentials to generate the signature.
Second, currently with the Web Meeting SDK, you can join any meeting, but you can only start meetings owned by the same account as the JWT App credentials.
More details here:
If you are still experiencing the invalid signature error, please email your signature to developersupport@zoom.us with a link to this thread and we will help you debug it. 
In the meantime, here are some similar threads:
https://devforum.zoom.us/search?q=invalid%20signature%20%23web-meeting-sdk
Thanks,
Tommy