First, constructs like "After (a) user(s)" etc., should be avoided. Unless your intention is to annoy and confuse the maximum number of people. When you put in those () brackets, you are creating little "either/and" branches for the reader on each one. After five or six brackets in such a sentence your user is likely to give you an anatomically unlikely suggestion as to what to do with your account creation process.
To resolve such questions, you decide who the audience is. Is it a single user at a time that creates an account? I expect it is a single user. Does that single user create one account at a time? Again, I expect it is. How many emails will that user get after creating one account? Suppose it is one email.
If there were an option for multiple users to gang together and create an account as a group, there is appropriate language to describe that. If multiple accounts can be created at a time there is language for that. And if multiple emails get sent after the account creation step (all too depressingly often the case) there is language for that. But let us suppose it is one for each of these.
So you could correctly write it the way you suggested. But there is a possibly better choice. Use "you" instead of "a user." Indeed, I would say you should write all user instructions in this fashion.
- After you create an account, the system will send you an email.
This removes the ambiguity of "a user" and the passive "will get."
