U have to form a party by Click "S" (Skill) on ur keyboard, double click [~FormParty] n click "OK" on the approved window, select the char u wanna invite, pop-up char names window appeared on the top of ur screen, mouse right-click inside pop-up char names window, select "Party Invitation", waiting for response from the char u wanna invite. If she/he approved n then done. Invited char will be on ur party list.
kikijo's post in How to Add people to my party? was marked as the answer