If you want a transfer to have an assistant, first of all you have to go to Admin > Transfers > Assistants in order to define one.

Then, in the transfer you want to add the assistant to, click on Assistants from the list of functions.

On the left will appear all the assistants loaded in the system, you can then select the one to activate and define on the right, its cost and whether it is compulsory. This means that during the booking phase the client will be informed of the additional cost in case it is compulsory, otherwise they will be asked whether they want to book it or not.