I stumbled across this bug this morning while creating users on a new 1.8 system;
Go to Admin -> Users
Select "Users" from the drop down
Click on a user's alias (first column)
add some groups to this user
save
Now notice that those groups are properly assigned to that user.
Select another (different) user
add those same groups to this user
save
Now notice that the second user has those group assigned, and the previous user does not have those groups.
Apparently assigning groups from the User panel only allows a group to belong to a single user.
Go to Admin -> Users
Select "Users" from the drop down
Click on a user's alias (first column)
add some groups to this user
save
Now notice that those groups are properly assigned to that user.
Select another (different) user
add those same groups to this user
save
Now notice that the second user has those group assigned, and the previous user does not have those groups.
Apparently assigning groups from the User panel only allows a group to belong to a single user.