Welcome to Android Central! This has been a deficiency in the Gmail app for a while. If you receive an email as part of a mass email, you can Reply All and send it back out to the group. But you can't create a new group to forward it to.
The only solution I've found is an app like Gmail Group Share, which works pretty well, although it has its quirks. If you try it, make sure you select both System Groups and Invisible Groups to be shown.