Peter,
Not all hearing aids have T-coils. A survey of the members would be prudent. They can use boxes about the size of a box of Marlboros with headphones, but most folks don't like being seen wearing hearing devices.
So you'll use a floor stand for the mic? If so and they'll be talking at a table, I like a stand with a small arm for poking it toward them. Guitar Center sells them over here for about $20 USD.
The mic, why not a good old SM58?
If you have the budget, maybe a Sennheiser 431.
Regards,
Ty Ford
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