I can think of a couple logical places to meet up. Anyone diving the south sites will be on the one main road going north past the airport and once you get north of Capt's Don's the same is true. At the south sites if you dive within one or two sites of your friends, you'll see them, most of the coastline is very flat with few trees. Only the north is hilly.
However if you go too far north, the road becomes one-way and you have to drive all the way around through Rincon back into Kralendijk from the NE. About 20mins. driving.
A couple of meeting options: The Lovers Ice Cream/Chat n' Browse parking lot by the rotary just south of the North dive resorts. It's right off the main road, there's also a small market and ATM at the bank next door. Chat n' Browse rents phones/sim cards also.
If you park along the lagoon south of the Plaza Resort, your friends will drive right by as that's the only way to get to the south sites or back to Kralendijk. Bring apples/carrots for the donkeys.
If the cruisers aren't in town, there's a couple places you can park/meet there as well. Just East of town there's a couple of small parking lots - one's just across from a strip mall that leads to the KFC.
Also you used to be able to park along the water in some areas near City Cafe also - at least temporarily. But not when a cruise ship is tied up at Town Pier. You wouldn't want to be in town anyway with 2000 cruisers - it's a pretty small place. I'd suggest that if you even see a cruise ship in port, you go the other way.
If anyone's diving with Buddy's, both of their locations. The drive-thru at Buddy's is located just off the road and there's plenty of space to wait nearby, their tank pick-up at BelMar to the south has room for maybe 2-3 cars without impacting their guests, it's right off the main road going to all the dive sites south. All of the south sites are no more than 100' or so off the main road so you 'll see your friends trucks anywhere in that area. Tie something unique to the antenna so you don't have to drive off the main road to check - there's a lot of similar looking dive trucks on Bonaire.
Any of the dive resorts. There's good diving at Capt' Don's (Cliff/LaMachaca), Buddy's (Buddy's Reef), Sand Dollar/Bonaire Dive/Adventure (Bari Reef), Eden Beach (Front Porch?), Divi Flamingo(Calabas Reef) and maybe the Plaza(18 Palms).
If you want to carry a phone, there's this:
Witz Phone Locker. Not sure if it's 100% Waterproof at depth. I've used their smaller case for about 5 years now with no leaks but mine is quite a lot thinner - not sure if that would be a factor.