Great idea to donate plants, but people will probably want to buy/trade plants as well. If enough people want to do that, try starting a separate "Plant swap" thread where people can post what they're looking for and what they have to offer
Also, it might be a good idea to start a separate thread for getting the day and time down. Let people post what times are good for them, and from there you can set an "official" time. Just a lot more organized that way
If the meet takes more than two hours, another idea is to have refreshments ready at the LFS, like drinks or snacks. Try to have someone responsible for getting this ready. You can set up something like $5 (through paypal?) gets you some munchies and a drink, or maybe two slices of a large pizza and a drink if that's more appealing.
Lastly, as mentioned above, it's a good idea to start another thread so people can post whether they will be going or not. That's for the sole purpose of getting an estimate of how many people are going to be there. Almost seems unnecessary, but it'll be really easy to keep track of who's coming this way
Yeah, it might seem like a lot of threads and might sound cluttery, but having everything in its own separate thread helps a lot, especially for the ones spearheading the meet.