Guests can join a meeting via a Teams link or by dialing-in using a unique meeting PIN.
Applicable to both guests and users a part of the organization, a meeting can be joined 5 minutes early or jump-in anytime within the meeting duration.
When joining via a Teams link, a guest will have the option to download the Teams app or join via a web browser. The quickest way for a guest to participate is using the web browser.

Web Browser

App

If trying to join meeting directly after downloading app, Microsoft will skip the sign-up step.
If a guest wants to use more of the free version of Teams features, they will have to create a MS account.
To use the free version of Teams desktop app, guests will have to sign-up for a free Microsoft account and are prompted to create one as part of the process when opening the Teams app after completing ~~it is downloaded~~.

Missed calls to a guest to not go to voicemail
As a side note for clarity, adding an external participant to a meeting is not the same as granting external participants access to an organization's Team. For more information on granting users access to a Team, read here.