SQLAlchemy: filter by membership in at least one many-to-many related table

Using SQLAlchemy 0.7.1 and a MySQL 5.1 database, I've got a many-to-many relationship set up as follows:

user_groups = Table('user_groups', Base.metadata,
    Column('user_id', String(128), ForeignKey('users.username')),
    Column('group_id', Integer, ForeignKey('groups.id'))
)

class ZKUser(Base, ZKTableAudit):
    __tablename__ = 'users'

    username   = Column(String(128), primary_key=True)
    first_name = Column(String(512))
    last_name  = Column(String(512))

    groups = relationship(ZKGroup, secondary=user_groups, backref='users')

class ZKGroup(Base, ZKTableAudit):
    __tablename__ = 'groups'

    id          = Column(Integer, primary_key=True)
    name        = Column(String(512))

Users can belong to multiple Groups, and Groups can contain multiple Users.

What I'm trying to do is build a SQLAlchemy query that returns only the Users who belong to at least one Group out of a list of Groups.

I played around with the in_ function, but that only seems to work for testing scalar values for membership in a list. I'm not much of a SQL writer, so I don't even know what kind of SELECT statement this would require.