User | Group | Expires
------+-------+------------
 1    | 1     | 2004-11-11
 2    | 1     | 2005-01-01
 3    | 1     | 2003-12-31
 3    | 2     | 2005-01-01

SELECT User FROM Subscription WHERE Group = 1 AND Expires > now();
# 1, 2

SELECT Group FROM Subscription WHERE User = 3 AND Expires > now();
# 2