The mishna says that if there is a group of witnesses and just one is a passul eid, the entire group is invalidated. R. Yossi and Rebbi seem to argue in the mishna if the entire group would be passul, even if the one person in the group who is passul didn't intend to be a witness. We pasken like R. Nachman who holds like Rebbi that the group would only be invalidated if the passul eid intended to be a witness. But, Tosafos asks on Shmuel who paskens like R. Yossi, how can we ever allow a mesiras ha'get or kiddushin to be performed in a large crowd that contains relatives - one relative even if he doesn't intend to be an eid can invalidate the entire group? From this Tosafos says that according to the conclusion of the gemara, a passul eid can only invalidate the group if he actually comes to court to testify, but by simply watching it will not have an impact. Therefore, in a wedding hall, even if a relative would intend to be a witness, he wouldn't invalidate the group since he never comes to testify. The Rosh quotes the Ramban who holds that only when the pessulim intend to be part of the group with the ke'sherim, and the ke'sheirim with the pessulim, do they combine to be one group. But, the Rosh himself rejects this approach and holds that if they both testify in beis din, it isn't relevant whether they intended to be part of a group together and the entire group would be invalid.
The Rosh himself (to explain the tosefta) writes that the din of one passul invalidating the whole group only applies when they are all "invited" to be witnesses. But, if the lender would designate witnesses, the fact that a passul eid will also be watching and intending to be an eid, it won't have any impact and wouldn't invalidate the group. Although the shach (c.m. 36:16) holds that designating witnesses doesn't work when the passul eid watches with the intent to be an eid, the ketzos (36:6) rejects the shach based on the explicit language of the Rosh in our sugya.
The sevara of the Rosh seems to make sense by a loan, but not by kiddushin. A loan is in control of the lender, therefore it is logical to say that he has the power to pick and choose his witnesses. Therefore, the designation of the lender works, as the Rosh writes. But a murderer isn't in control to designate his witnesses and decide who should and who should not be witnesses, rather everyone who watches is potentially a witness if they intent to be one. Similarly, by gittin and kiddushin, why should it be in the control of the husband to pick his witnesses (although the sevara of Tosafos that they would only invalidate the entire group if they actually testify in court, would still apply)?
The Ketzos HaChoshen (also in avnei mi'luim) offers a very interesting peshat and holds that for gittin and kiddushin this entire din doesn't apply. He proves from the fact that kiddushin isn't binding if the bride and groom are unaware of their existence such as when they are hiding behind a fence, that they must have an awareness and acceptance of the eidim for them to qualify as eidim. Therefore, the entire din of נמצא אחד מהן קרוב או פסול עדותן בטלה wouldn't apply to kiddushin since the chassan and kallah would obviously only intend to include those who are kasher. The only cases where we would ever have an issue of נמצא אחד מהן קרוב או פסול עדותן בטלה would be when eidim aren't necessary to make it go into effect (i.e. gittin and kiddushin) and the eidim aren't designated. But by kiddushin and gittin even if the eidim aren't designated, the passul eid wouldn't have the ability to invalidate anything since only those recognized by the chosson and kallah can be eidim, and they would surely only have in mind to include valid witnesses.
We see from here that there is absolutely no reason to be machmir for the chosson and kallah to announce that they are designating the eidim: 1. because they are essentially announcing this when the eidi kiddushin are called up. 2. even if they had to designate eidim, they wouldn't need to announce it to the crowd. 3. they are never testifying in court (so according to tosafos there isn't any problem). 4. for gittin and kiddushin even if they didn't think about specific eidim, the passul eidim are automatically excluded since the chassan and kallah must be aware of the eidim for them to qualify as eidim (ketzos).