The word alistaréis is the future form of alistar in the second person plural.
See the full alistar conjugation

alistar

Os alistaréis en el ejército.
You'll join the army you choose.
Want to Learn Spanish?
Spanish learning for everyone. For free.
SpanishDict Premium
Have you tried it yet? Here's what's included:
Cheat sheets
No ads
Learn offline on iOS
Fun phrasebooks
Learn Spanish faster
Support SpanishDict