The Guild CBA is a "union-shop" contract that requires all employees to join the Guild it's not optional. However ...
The initiation fee is routinely waived for anyone at the studio at the time the crew votes to go union. The waiver is offered to all employees under the Guild's jurisdiction, not just those who supported the Guild or signed cards. The initiation fee is equal to two weeks of the union minimum in one's job classification, so it generally totals around $2,000 to $2,700. That's obviously a big perk for being at the studio at the time it goes union.
If there are a large number of inactive members at the studio, the Guild will also consider waiving some of the reinstatement fees, and applying to the IATSE to waive their portion of the reinstatement fees as well.