At some point fairly early in the development of your PhD research, you should form a PhD Thesis Committee to provide council and guidance. Typically, a PhD Committee consists of the research supervisor and two other faculty members. The two additional committee members are typically faculty members from the Department of Psychology. However, it is possible to have one of the committee members be a faculty member from another department at the University of Waterloo.
Although the norm for a PhD Thesis Committee is the supervisor and two other committee members, it is permissible (but somewhat unusual) to ask one more faculty member to join the committee, making a committee of four. If a student has two faculty members serving as co-supervisors, they count as just one person on the committee and two additional Department of Psychology committee members are required.
Another possible (but unusual) variant is to include as one of the committee members a faculty member from another university. Such a person would be required to get an adjunct appointment at University of Waterloo to be part of the committee, which would require the approval of the Dean of Arts. Contact the Associate Chair, Graduate Affairs, if this possibility interests you.
When the thesis is complete but prior to the defense, it will be necessary to add two further members to the committee, an internal/external examiner and an external examiner. For further information, see Guidelines for the PhD Thesis Defense.