First off, if you're unsure of riders' ability levels, arrive especially early to the meet-up point. Position yourself so you have a good vantage point of wherever riders will turn in off the street, and far enough from the entrance so some slow-speed cornering will be required to get to your assembly area. This gives you an opportunity to watch as they signal, pull in, and navigate the parking area on their way in. Also, observe how they park (are they comfortable backing a bike into a spot, etc). Probably not going to be able to sort out the medium to strong riders with this, but you might get a chance to note if any of the group are in that 'less than proficient' category, and hopefully get them in the middle of the group as appropriate.
Unless you're publishing the route in advance, I would recommend you lead, since you'll know the route. As riders pull in, ask if anyone has experience with group rides, and if anyone has been a road captain before. Not meeting anyone beforehand it can be a bit of a crapshoot, but I've usually had decent luck finding someone competent to act as the trail bike. Make sure they have a cell phone and exchange phone numbers with them in case they need to contact you should someone have a flat, etc, etc. With a church group, your odds are decent that someone will have some reasonable experience with group rides. Also, since you're not going to know the group's abilities, I'd recommend a shorter ride (<100 miles)... although I'm with the previous poster about wanting to RIDE, not hang around and talk about it.
Standard pre-ride briefing for groups goes more or less as follows (I assume you'll have a motley assortment of cruisers, sportbikes, etc):
-First off, everybody ride your own ride. This ride is for fun, and it should stay that way for everyone. If you feel like you're being rushed by someone behind you, move to the right and wave them past. The trail rider will not pass anyone, so if you're second to last just ride at your pace and the trail will follow up.
-Safety is paramount, so everyone needs to ride courteously and obey all traffic laws. (At this point being military I usually mention that any military folks not having proper PPE will not be allowed to ride with the group unless they go home and get the right gear. I usually put that out in any announcements / flyers beforehand so it generally isn't a problem).
-Explain hand and arm signals that will be used for the ride. Left and right turns (even with turn signals), Slow down, single file, staggered formation, and "I have an issue and will be pulling over, don't follow me" are usually sufficient. Also usually throw the 'cop' signal in here for some comic relief (someone inevitably doesn't know what it is).
-Cover procedures for what the group should do if separated at a stoplight. Whether this means the lead element stops, slows, or whatever, make sure everyone knows what to expect.
-Cover procedures for lane changes (interstate or state highways) if the group is large enough.
Anything beyond those basics (unless I've missed something obvious) would generally fall in the 'advanced group techniques', and require either experienced group riders, multiple road captains, groups that have ridden together on multiple occasions, or some combination of the those.
Understand that you're going to have to set a constant pace so the group doesn't slinky too much, and you'll need to allocate about 50% of your attention to your mirrors in the twisty stuff to make sure that you're not pushing the group beyond riders' experience levels. I've been on plenty of group rides where I had to drop down to what I felt was a snail's pace because I realized that the guys behind me were doing okay, but were having to push it to keep up. While it's not quite as much fun to have to slow down, it's safer for everyone. However, it does feel rewarding to know that you not only have the skill to ride much faster, but can also recognize that someone is approaching the edge of their performance envelope trying to keep up.
Let us know how it goes!