"Hey guys" is a common salutation, but it's not gender neutral. People who aren't men can feel excluded when addressed as "guys," even if that is not the intent.
Instead of using "hey guys," consider one of these several alternatives:
The aim of heyguys.cc is to encourage the use of inclusive language in online and in-person communication.
Feel free to add the following to your email signature:
Q: How can I use more inclusive language? A: https://heyguys.cc