If your dependants are extending their visas at the same time as you, they can be added to the your online application form.
If your dependants are applying separately from you, they will need to fill in their own form online. You should book a place on a visa workshop or, if your visas will expire in the next month, book an appointment with an adviser. The adviser will show you how to make a dependant application at the workshop or appointment.
Read the ISAS 'Tier 4 Dependant' leaflet for more information.
Web link: http://www.swansea.ac.uk/isas/immigration/tier4dependants/