I'm having the same problem too, from 6.5.2 -> 7.4.1.
For my users, when looking at their accts on the permissions tab, 'Member Status' is correctly showing as 'user', but under 'Group Membership' nothing is checked.
Under the 'Group Management' tab, it says I have tens of thousands of Group #3 Moderators, and 0 Group #4 Users.
I'll open a trouble ticket also. A quick mysql command is probably all I need.