Can non members speak in Sacrament Meeting?
Thanks for your question. In describing sacrament meetings, Handbook 2 states:
Selecting and Orienting Participants
Selecting Participants. The bishopric selects members to participate in sacrament meetings.
So it is expected that only members will speak in sacrament meetings. Note that during fast and testimony meetings, the Handbook states (emphasis added):
After the sacrament, the bishopric member who is conducting the meeting bears a brief testimony. He then invites members to bear heartfelt testimonies and to relate faith-promoting experiences.
So even our fast and testimony meetings are intended for the Saints to bear testimony to each other.
However, I have witnessed several occasions when non-members address the congregation and even bear testimony. I have never seen a bishop prohibit this behavior. So while it is non-standard for non-members to bear testimony and not encouraged, I do not believe it is prohibited. Such a decision would be at the discretion of the bishop or other presiding authority in a fast and testimony meeting.