I am the second author of three, and the only one who does not have an active membership. I have little need for the other services provided by IEEE, as I already have access to XPLORE through my work.
Nope, and I doubt anyone would notice or care.
Everyone can be a member, all it takes is a bit of money.
As mentioned, I doubt anyone cares, except if you or co-authors are Fellows, then it'll carry some weight. It might convey the following message: the paper has been vetted and informally approved by esteemed member(s) of the IEEE and therefore all reviewers+editor should accept the paper with a grain of salt.