On the friends tab the join game option is locked no matter if they are in a game or party.
On the recent players tab the join game option is unlocked but I have never tried to use it.
On the clan tab it is also unlocked but gives private party error when attempting to join, otherwise you can join when they are in a game.
I want to say that this is simply a miscoding error like the hitboxes for sniper rifles with chrome lined barrels, because otherwise it is pointless to even give the option "Join Game" in the first place.
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I haven't had a single issue with joining anyone(through invitation), it's just the tedious nature of consistently having to invite people to a party. It appears as though you are supposed to be able to join via the friends menu without invitation but the option is completely unavailable. The option to "Join Game" then makes no sense if the only method of joining is by invite, hence the question "Is there anyway to change party privacy?" because everyone I play with is in my clan but joining is impossible(through the join game option) with a permanent private party in place and no option to change it. If I didn't explain it properly, my apologies.