You can check for users in each user group using the standard user search, just select the group.
There is no merge function and it's not recommended to move users via SQL queries due to the permission combination value.
If you delete the user group, the members will default to the normal registered user group, if they aren't already a member of it.
User groups 1-4 can't be deleted as they are the default groups.
- 1 - Unregistered / Unconfirmed
- 2 - Registered
- 3 - Administrative
- 4 - Moderating