How should that work out? youl invite everyone to a meeting, asking them and put a contract - which you have already prepared in advance with your lawyer - right in front of them on the table and put them a pen in their hand and force them to sign? after they are your "friends" or "partner" for long time? and if they think this is not very friendly behavior, you will quit friendship if they dont do it?
i would never sign something like this, there is zero benefits for me, i only could loose if something happens and something gona leak someday and i maybe mistakenly got suspected and i may would have to take financial damages out ofthis NDA... it would also reduce my trust in this friendship if i would get asked for something like this...
its like the idea of setting up a marriage contract AFTER the marriage already happend...
this would be a hard hit into any already established trustfully realationship, in my opinion.
i am sure maybe superstars do this at the beginning of a serious relationship so there will be no blackmail potential for both partys and the career after a split up will be safe without washing any dirty laundry out in public and media.
otherwise than that i would just expect real friends to keep that topic secret if i would tell them about, and if they not do, they are worthless anyway and it will better to cut them off asap...
If you get famous above a certain level i think its impossible to stay 100% anonymous anyway.