How do I track a visitor between my different domains?

We only use what are known as first-party cookies, that is to say, the cookie that is added on the currently domain you're on. A contact's profile can be updated from multiple domains if you have two or more domains registered within your Triggerbee account and the same tracking code is implemented on each site. You also need to allow this url or url:s under "Allowed URLs", this you find under Settings in your TB-account. 
So, as an example, if you identify a visitor with an email address on both site 1 and site 2, only one contact will be created in Triggerbee which will include data and events from both sites. 
Please note: Identifying a visitor in one domain does not make it automatically identified on the others.