Is there a variable which holds id of users which used a bot?

There is no variable like this, but for exemple you can store the user id at the beginning of a conversation in a table , and check if this id is already in the table or not

Is this user id in event.userid?

yes you're right !

Can I access that table like an object? For example tableName.userid === event.userid

Hum you need to loop on all the entries of the table and check if one is equal to event.userid