I wonder would if it be easier to add a table in the database and assign
so the two usersID's from both tables are relationsional or linked (not sure on this)
on your backend create a list of groups A, B, C, etc.. with checkboxes so they are displayed for each user, then you can select the checkbox for that user and it corresponds to the groupID number in your Mapping table. so A=1, B=2, etc..
assign a value to each checkbox from 1-26 (for A-Z)
so for example user A (userID = 231) wants to be in groups B and C, you check B and C, and it inserts into your Mapping table
userID = 231
then link the groupID in your access rules page.
I'm just throwing out an idea