How can I allow Guests to participate in a chat activity?

Simply create a visitors role with the capability mod/chat:chat. 

To create the role:

• Go to Settings > Site administration > Users > Permissions > Define roles.

• Click the "Add a new role" button.

• On the add a new role page, give the role a name e.g. 'Visitor' and a short name.

• Give the role a description (optional).

• Select the contexts where the role may be assigned e.g. 'User' for visitor role.

• Set permission as required and allow mod/chat permission.

• Scroll to the top or bottom of the page and click the "Create this role" button.

Once done, create an account and assign it the role of Visitor. Guests can then share this visitor account.

Other permissions can also be granted to the role. This purely depends upon how much autonomy you are willing to provide your site visitors.