By "flag" I assume you mean the check box?
Taken from the official PSO2 site on "Party Management" (use Google Translate):
http://pso2.jp/players/manual/prepar...ty/management/
I think the check box that you checked was for "1 Play Only" (for the"Play Style Option"):
And using Google Translate:
"Select the play style of the party. This play style is information for participating members to the last, it does not limit participation itself as a function.
By entering a check in "Welcome to join", you can set welcome welcome. When you set a welcome welcome, it will be displayed preferentially in the quest selection of the quest counter. If the party setting welcome has reached the party member's limit by the accompanying partner, the partner will return by players participation.
If you decide to dissolve a party in Quest 1 play, you can tell the party member and the user who is about to participate by putting a check in "1 play only"."
So in other words, by checking the box named "1 Play Only" you're basically just telling the other players that you're only going to do the run 1 (single) time, this doesn't prevent you from having others joining your party.
However, if you want NO ONE to join your party, then create a password (just like what you said above) and check the 2nd box (bottom one) in the Password option.
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