Best way would probably just to get rid of everyone in group 5 and then if you have MySQL 4.1+ you can add everyone to the user's group that isn't already in a group.
delete from ubbt_USER_GROUPS where GROUP_ID='5' and USER_ID > 1
and then:
insert ubbt_USER_GROUPS(
select ubbt_USERS.USER_ID, 4
from ubbt_USERS
where ubbt_USERS.USER_ID not in (
select ubbt_USER_GROUPS.USER_ID
from ubbt_USER_GROUPS
)
and ubbt_USERS.USER_ID <>1
)
On another note, I need to figure out why this is happening for some people. Out of all the ugprades I've done, I haven't had this happen.