I'd do away with the whole extra click of making someone 'accept' or 'decline' the invite. Just have the two buttons in the dialog, on the first screen. Don't make me read, then click, then read again, then click. There is no logical reason for that extra step at all.
The button on the right should be the 'continue' or 'ok' button and the one on the left should be the 'cancel' button. Don't use text for 'cancel' or 'decline', just make them buttons. The buttons should be right aligned at the bottom of the text. This is how the user expects things to be based on UX experience of their operating system, don't confuse users by changing it up.
I just spent 5 minutes in Balsamiq mocking something up for you . I'm not super happy with the decline button, I might play around with it in different areas (or ideally get rid of it all together). The point being that there is no need for the 'accept' step, since they aren't accepting anything, they are signing in or creating an account.
By the way, for my site, we got ride of the whole create an account process entirely. We just have sign in. User clicks sign in and they have the option of using their FB account (with the bare minimal permissions) or using a BrowserID account (which only requires an email and password).
Keep it as simple as possible.
A nontrivial number of people will come up, read the sign, and promptly try to open the OTHER door, see that it's locked, and then walk away to try and find the "other door" - generally trying to push open the out doors. As it turns out, they all read the sign as saying "Please use other door".
(Occasionally they'll try to claim that the sign is wrong. There's a certain schadenfreude in the looks on their faces when I convince them to reread it more carefully.)
So, we removed the handle for the door that's always locked, and took down the "Please use this door" sign. Now people just walk along the out doors, pushing on them to try and find the one that'll open.
If you were in front of the door that is open, why say 'Please use this door.' They will. You don't have to say so. People generally do not walk up to the door they do not tend to use. Hence why they assumed that the sign said 'Please use other door', since that is 1) what is most often on the doors and 2) it's illogical to have a confirmatory sign on the open door.
the users that aren't able to fill out the form they will be unable to continue until they read. they will start looking for the "sign up" button. make it big and obvious for them. still, nobody has made any decisions. the user takes the only path available to them.
'The software isn't performing the analysis'
"Okay, what does the dialogue box say"
'The data file is corrupted'
(we have no such dialog box)
"Hrm, I haven't seen that box before, can you read it out word-for-word to me?"
'The. Data. File. Is. Corrupted'.
"Hrm, I've not heard that before, could you please spell out the words letter-by-letter so I can get it exactly right for the developers to look at?"
'Sure: Y-o-u-r s-e-t-t-i-n-g-s a-r-e n-o-t c-o-n-f...'
Even when specifically asked to read the message out word-for-word, endusers find a way to screw it up.
I got several emails from users who asked me "How do I register? I looked all over the front page and found nothing."
In hindsight, I wonder if maybe I needed to make it look like a button, or if these particular people were beyond saving and wouldn't have bothered to read the text on the button anyway.
AdBlock Plus did it for me, and delivered me a blank page.
Your users will read — but only enough to feel they have an understanding of a situation before they take action.
I read quite a lot of text online, and I certainly have a different style of reading a web page versus reading a book. I'd be fascinated to know how efficient scanning is after years of practice online, in terms of information consumed that you're able to recall. 28% is surely too low for adequate comprehension, though.
If you pour a lot of care and attention in to some copy or instructions. They won't even not read it, they will flagrantly disregard it.
If you fluff up some filler text to fit a space, they will go through it with a fine tooth comb and hold you to the fire over it.
In my experience, you can't win.