For Cite Them Right Harvard you would include up to three authors in your in-text citation, for example:

One of the key studies on this subject (Hill, Smith and Jones, 1997) explains…

If your reference has four or more authors, you can then use et al. (meaning ‘and others’) in your in-text citation, for example:

Research by Harrison et al. (2003) shows that…

For Cite Them Right Harvard, you need to record all authors in your final reference list or bibliography. You can only use et al. if there are 10 or more authors.