static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(
    Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)
{
    if (null == valueSelector) { throw new ArgumentNullException("valueSelector"); }
    if (null == values) { throw new ArgumentNullException("values"); }

    ParameterExpression p = valueSelector.Parameters.Single();

    // p => valueSelector(p) == values[0] || valueSelector(p) == ...

    if (!values.Any())
    {
        return e => false;
    }

    var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body,
		Expression.Constant(value, typeof(TValue))));
    var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal));
    return Expression.Lambda<Func<TElement, bool>>(body, p);
}

Usage is similar to the following:

var assignees = _entities.Admin_Users.Where(BuildContainsExpression<Admin_Users, int>(e => e.Admin_Department_ID.Value,
                departmentsToReturn));