IEEE (Institute of electrical and electronic engineers) are more concerned with engineering than computers, engineering institutes usually offer their members access to lectures and learning material (also access to a network of other professionals).
BCS (British Computer Society) you can find out what they offer there members here..
IMIS (Institute for the management of information systems) is much the same as BCS, in so far as they offer you a membership and exams that you can take to get accredited to their standard.
IAP (Institute of analysts and programmers) basically offers the same and the previous two.
the best way to describe what these organisations do for you is o quote this from the IAP website
Membership of the Institution is an endorsement of your professional competence by your peers in the industry. It can advance your career and boost your business. It gives employers and clients the confidence that they are dealing with an accredited professional.
Basically, they are just a boast, the trouble with most of those organisations is that they are practically unheard of! IEEE is the most recognised, but it doesn't exactly fit the bill of a computer related qualification.
I've been a member of the IET (former IEE) for some three or four years now.
I never add the professional association letters of the organisation after my name, (I don't use my degree letters either)
bascially all these memberships do is give you some letters after your name, (though I'm not sure that all of those ones actually do that. they offer you access to their resources, though you can find most of those resources with google anyway.
most offer a way to becoming a chartered professional, of course not that that chartership is worth anything if it's not a recognised field of chartership, and of course they cost money.
you should be wary of an institute that has very low entry requirements, (i.e you only have to pay)
most institutes, have entry requirements that are well suited for students, i.e you have to have a degree or equivellants work experiance and be able to demonstratte good knowledge.
some institutes are a little more picky than others, for instance to join the IET you have to have a degree from an accredited university, that is to say that some universities don't offer degree courses good enough for them to want you to associate yourself with them!
almost all institutions require you to have a supporter when applying who can vouch for your abilities, this will almost certainly be a line manager/boss or in the case of a student applying straight after uni will likely be your old lecturers (assuming that you don't have a job yet).
I also couldn't help but notice how far apart the disciplines that you were looking at go, I mean there is quite a range there, information system management, engineering, programming. perhaps you should try to decide on what area of the field you are looking to really specialise in and go for affiliation with whatever institute best represents you.