This addon will assist you by:

Occasionally putting a list in your chat box of all the people that recently left the warband, together with how long ago they left, and which party they were in

The names are right clickable, so you can do a group invite straight from there

If a player rejoins the warband (whether you are the one that did the invite or someone else doesn't matter), he is automatically moved to the party he was in before

If a player cannot be moved because that party filled up in the meantime, there is instead a message informing you that parties need to be fixed
The addon currently has no configuration or slash-command. If you want to configure how it runs, you can manually edit some values in the file "rh_const.lua" on the top however. What's currently configurable there is:
* Whether to display anything if you are not a warband leader or assistant (HIDEWHENNOTLEADER)
* Every how many seconds a list of the missing players is printed to the chat window (value REPEAT)
* For how many seconds a player is remembered after leaving until assuming they will not return (value DISPLAYTIME)
* Whether or not to automatically move players between groups (value MOVEPLAYERS)
Since this is a rather specialized addon I'm not sure if it's worth spending the effort to code a UI for it since that seems to be rather complex, but maybe I'll have a look at it in the future.