You're going to want an IT Manager well before 100 people. I handle systems integrations & cybersecurity compliance for a 15 - 20 person PR, marketing, & design firm and I am constantly busy with everything from major operations initiatives, major systems setup, and being available for end user IT issues.

That being said, get a Mobile Device Manager (expect a year to get it configured to where you want), get a company password manager, get a cell phone plan figured out (expect to spend $$$$$ on phone systems).

All of these systems will require significant time for set up & deployment, especially if you are doing it in such a way as to grow your compliance posture at the same time.