If everyone had a wireless mic, you would need many different frequencies. You would need a multi-channel receiver, monitoring all frequencies, and combining all signals to one hearing aid. This would be unwieldy although it is ideal.
Networking the units is a worth a try. I don't know how possible it would be to have more than one person talking, while your father receives all talkers simultaneously. I picture momentary dropouts while the network tries to keep up.
One of my favorite people in life was an old gentleman who was hard of hearing. He purchased a stereo hearing amplifier at Radio Shack. It combined two mics and amps in one unit.
He hung it around his neck and put the headphones on. It worked fine when he talked to anyone standing in front of him.
Sometimes a bunch of us would be at a table, talking. He would hold up the mic unit toward whoever was speaking. It is important to get the mic close to the source.
He also got a stereo extension cable. Then we would pass the mic unit around to whoever was talking at the moment. This worked fine as long as we reminded each other to 'pass the mic'. We had to be careful, because there were times his ears got a pounding if the mic unit knocked against anything.
It's important that it was stereo. Binaural amplification conveys a more complete sonic image, which is crucial to picking out a voice amid surrounding noise. A monophonic hearing aid is less than optimum. Both ears need to receive an amplified signal.
Someone may suggest a walkie-talkie (for wireless convenience). The ones I've seen are monophonic, which means it will be less than optimum.
Nothing will work flawlessly. Elderly ears usually develop a problem hearing high frequencies. Typically a hearing aid amplifies this range, and it helps a lot. Hearing aids can be adjusted to have a different frequency response, besides adjusting volume. See if you can do this on your own.
By the way, there is a frequency spectrum analyzer in Windows Media Player. Choose the visualizations called 'Bars and Waves', and select Firestorm.
Watching it, you will spot the frequency ranges that are activated when a man is talking, when a woman is talking, when speaking the letter 'S', etc.
Play with the built-in equalizer and you can alter frequency response.
All this may help you figure out what adjustments will let your father hear better.