This can be quite tricky with all the options there are. I'd suggest looking into other skills and how they are applied, for example things like assist buffs (Quickstep and such) are: "EXT_ANOTHERWITH WUI_TARGETINGOBJ SRO_DIRECT", the three slots, where you can define how the skilll is applied.
While the Ringmaster skill Geburath can be this: "EXT_TROUPE WUI_NOW SRO_REGION", different from the base in case you're wondering.
The first is a selfbuff, but can function as a shared one, if you target a other player. Second is a partywide buff and a selfbuff in one - doesnt work without a party tho.
As far as i know, to make the Partywide buffs work without a party, you would need to chage the source a little (Ctrl.cpp, in DoApplySkillEx).