I've been playing around with Hustle hoping for a simple form.
However then I discovered that there is no option to alter button / field text. I found reference to using Ultimate Branding.
My site goes down when I try to activate U.B. - It appears to get stuck in activation using the WPMUDEV dashboard and I have to delete the plugin to get back into the site.
PS: I have to admit that I find myself disappointed with the plugins. I decided membership was a great idea, but I give a plugin a go, but then find it doesn't have something that seems simple and I have to look elsewhere. So there's currently nothing I'm actually successfully using.