Handling large number of OR/IN conditions

We have a database report function that seemed clean when the number of
users was small, but as the number of users grow, I was wondering if
anybody had any good ideas about how to handle OR or IN for SELECTs.

The general scenario is that a manager runs reports that list all
records that were created by users under his/her oversight. So, when
the number of users is small, we had simple queries like:

SELECT field1, field2 FROM table1 WHERE creator_user_id = 'U1' OR
creator_user_id = 'U2';

But when there are thousands of users, and a manager has oversight of
100 of them, the OR construct seems out of whack when you read the query:

WHERE creator_user_id = 'U1' OR creator_user_id = 'U2' *...* OR
creator_user_id = 'U99' OR creator_user_id = 'U100'

I know it can be shortened with IN using something like, but don't know
if it's any more/less efficient or a concern:

WHERE creator_user_id IN ('U1', 'U2', ...., 'U99', 'U100)

How do people tend to handle this sort of thing? I suspect manager
reports against their people must be pretty common. Are there any good
tricks on how to group users like this? Unfortunately, group membership
changes over time, and users may report to more than one manager and
thus belong to more than one group, so we can't just have a
'creator_group_id' attribute that is set and then query against that.

Thanks,
David