When Acejavelin says use "feature" 5, he's referring to the SPECIAL (or feature) key, the one with the X in a circle. So to build a conference you call first party, then X5, then second party, X5 again, until you've called everyone. Then X5X5 to join everyone.
The system can also do "meet me" conferencing where everyone dials in and enters a conference code. For that to work you must have the CONFIGURATION ASSISTANT programmed in SYSTEM/DEVICES AND FEATURE CODES/ASSISTANTS. You create the conferences from one of the USER web accounts, plus each USER who has conference authorization automatically has a code that matches his extension created. So, dial the conf assist, enter code and #