Pedants - I need some help with abbreviations and apostrophes

[TheFallenMadonna]

I am editing the village magazine, and require some help with the WI's article. Could my life be any more thrilling?

So, the plural of WI would be....

WIs?
WI's?
WI?

WIs IMO

how the spring days must fly by for you

There are worse things to do on a Saturday afternoon!

I would go with WIs. There is no need for an apostrophe on plural acronyms.

Thank you.

Spring? Ha! Hail is beating against the window right now...