Let's take the example of a page at the end of an app signup process.
This page will allow the user to finish the process, so I will place a button labeled "Complete Your Registration" and in the same time it will allow the user to invite friends (by filling email inputs) to join this app.
So the idea to change the label to "Invite Friends and Complete Your Registration" when the user enter at least an email address in the form.
Is it ok to change the label depending of the user's choices?
Just changing the text of the button is not informative enough when changing its behavior.
Normally, people stop reading the texts when they get used to the system. A returning user adding email addresses for the first time, might send notifications unintentionally. Also, it might not always be clear to the user why the button suddenly changed. Your user case might require a certain approach, but generally there are better ways to provide two different actions:
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Is it possible to complete the registration without sending the invites, even if the invite field has been filled?
I think that instead of having a verbose button, you should simply clarify expected actions by including the following label above the invite email field:
Optional: Enter friends' emails below to invite them once you register
From a copy-editing point of view, conciseness is king. So, from bad to best, your button label should read:
In my view, this is completely fine - you're making it more clear to the user what action they are taking when finally clicking the button. The only thing you may want to watch out for is it might be visually jarring to have button labels changing as the user fills out a form. If it's possible this might happen (i.e., the button and most of the form are visible at the same time), you might want to add additional cues (i.e., a flash message) to make the UI change less confusing.
I think that you should leave the button as it sits. However, you should change the action that it takes once emails are filled out. Once they decide to invite friends, have the button take them to a confirmation screen indicating that by submitting the form, they are also going to be sending a notification to the email addresses they filled out (disabled textboxes or just labels). Then you should provide two buttons to go back and edit or proceed.
This gives a notice while giving the user time to confirm they actually want notifications sent out as well.
To some degree the answer, as usual, depends, it will depend to some degree on the skill set of your users. If they are intermittent users then I would argue against changing text as it scares them into thinking they have done something wrong. Robust users will take it in their stride. If you don't know then assume a dumb user rather than a smart user.
