It is your job to meet the needs of the business, even if it means embracing a solution they have chosen.
It's not them, it's you.
Why do business people sign up for Software-as-a-Service (SaaS) when you think you offer a perfectly good solution within your IT organization? Let me let you in on a secret: It's not them, it's you.
Business people sign up for Dropbox or Google drive because your file sharing is too hard.
They sign up for gmail or iCloud because you block attachments, limit their mailbox sizes, or don't support their phones.
They get free video conferencing services because they can't figure out how to get a toll-free number with your service or because they had one bad experience with it six months ago.
Those of you who know me know I could go on and on.
Here is the point: Your colleagues are signing up for these services because they need the functionality they get with them, but they don't know how to get that same functionality with what you provide.
When you discover someone has gone around your IT organization like this, take a deep breath. Talk to that person to understand why he/she felt it was necessary to use a different solution than the one you provided. If the person just didn't know about your solution, problem solved. You kept your blood pressure low, and you should have a happy customer.
If that person deliberately chose not to use the solution you provided, you have more work to do. Once you understand what he/she is trying to accomplish, you will want to make one of these choices:
In collaboration with that person, overcome his/her obstacles and transition him/her to the existing solution.
Grant that person a formal waiver until your capability can meet his/her needs. If you don't have a procedure to grant a waiver, this is a good test case for you.
Embrace what that person has done and secure it in a manner to make it acceptable for long-term use.
Shut that person down. If the solution compromises the integrity of customer, company, or employee data, explain why he/he may not do what he/she was doing, explain the acceptable solution, and offer training, even one-on-one training. But tell that person with no possibility of misunderstanding that he/she may not use the non-compliant solution any longer. Take this to your boss or the person's boss if you cannot agree. The integrity of the company is not negotiable.
Above all, remain open minded that points 2 and 3 could occur more often than you might want to admit. An end user, singularly focused on solving a specific problem, may devote more time than you and your team to scouting out possible solutions. Take advantage of that passion and effort and incorporate the best of it into your IT thinking.