If I log a call or meeting in a client's Communications tab, does the client get notified?

No. Logging a call or meeting in a client's Communication tab (log meeting or call) is purely an internal record for your practice. The client is never notified.

Attendees, whether staff or the client's contacts, just record who was on the call or meeting (for example 4 staff and 2 contacts), so you can track who attended. Adding attendees doesn't send a notification to anyone.