If you are receiving compensation (e.g. cash or equity) in return for advising a startup, then it’s considered a Category II activity, and accordingly, you need to annually report this activity to the university.
If you are not receiving compensation for your advising, then the activity is not likely to incur the time and responsibility required to merit annual disclosure to the university.
Here’s a good summary of the different categories of activity, and corresponding policies:
https://academicpersonnel.ucr.edu/resources/coc/ConflictCommitmentUCRsum...