My best quick guess is that a StartTime of NULL indicates an unused entry. Even if not, the code below may give you some ideas for the correct column(s) on which unused status is based.
--first create an index to speed lookup
CREATE UNIQUE NONCLUSTERED INDEX IX_Crews_StartTime
ON dbo.Crews ( StartTime, CrewID )
--WHERE valid only on SQL 2008+
WHERE StartTime IS NULL
--if on Enterprise Edition, uncomment "ONLINE = ON,"
WITH ( FILLFACTOR = 99, /*ONLINE = ON,*/ SORT_IN_TEMPDB = ON )
ON [PRIMARY]
--then do the actual lookup itself
SELECT /*@CrewID = */ MIN(CrewID)
FROM dbo.Crews
WHERE StartTime IS NULL
SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.