You can use the identify API call to identify your users.
_browsee('identify', '<user_id/email>')
The second parameter to this call can be any string. However, it's best to use database IDs or a hash which does not change when a user updates their information. If the identifying parameter is common along their journey, you can unify all the sessions of a user.
You can also send attributes to the identify call like
name and email are special attributes. When you send name or email in an identify call, you can see them in the session panel on the right while replaying a session
All the attributes you send via identify call can be searched for while creating circles.
You can also assign tags to a session. Like
_browsee('identify', '<user_id/email>',
{'$tags': ["Logged In", "Cohort 78435"]})
Like attributes, tags can be searched for while creating circles and are visible on the session replay screen. More about creating circles here.