Seats are only available with business Teams & Enterprise plans. Your plan admin is the only one who can add/remove seats from your organization's plan.
Failing that, your collaborator needs to obtain his/her own subscription to Photoshop.
The Photography Plan bundle is the most affordable at USD $10/month x 12 months.
Nancy O'Shea, Product User & Community Expert
Alt-Web Design & Publishing ~ Web : Print : Graphics : Media