How to join steel pipe for decorative purposes
Use pipe shorts (nipples), they're threaded on both ends and can be acquired in various foot lengths, and everything in between a 'close nipple' (~0") up to 18". But you should price check the cost of a threading verses a short, and which one would be less of a headache for you.

Or use EMT. For esthetics, use compression fittings and paint everything black.

Do not file away some of the OD and jamb a steel pipe into a fitting. You do not want it to fall on someone. For that same reason, do not use adhesive.

Although it would work, if you knew how to weld and owned a welder, I don't think you'd be asking this question. 

I'm hip to decorative steel pipe, but only if it's threaded together. Using couplings that don't belong on steel pipe would scream "DiY project". And joining pipe together poorly would be poor form. If you're going to hang 3/4" steel pipe above my head, I expect to be able to do pull-ups off it. If you go with 3/4" EMT, then whatever. But if you do you'll lose the decorative aspect; EMT is simply cheap, easy, and functional. Where as with pipe, you can always still see some thread and tell it's robust - that's part of its charm.