Currently, you can only have one waypoint on a route and you cannot pre-plan the route. For multiple cars, Waze does work nicely for being able to know when each will reach the shared destination. However, you are still limited to only one waypoint. If it isn't a big deal, just do a trip to the first destination, then one to the next, and so on. If you're not all going to be using the same waypoints or you are not going to wait for each other at each waypoint before continuing on, then you may just want to set Waze to the final destination and share that and then put Waze in the background while using another program to navigate through the waypoints. That will mean your time to reach the destination will change whenever you go another route to reach a waypoint, but once everyone is on the final leg of the trip, you'll all have accurate information on when you'll arrive.
And yes, multiple waypoints and pre-planning of routes have both been requested many times and should both be on Waze's To-Do list. No idea when they'd be added, but they probably will be added at some point.