The sweep’s overriding responsibility is to make sure that no hiker is left behind or lost.
The sweep should be the last hiker in the group. The sweep is responsible for communicating with the hike leader any problems with the hikers near the end of the line. For example, the sweep might ask the hike leader to slow down, stop for a break, or help treat a blister.
The sweep should also point out to the hike leader anything that the hike leader is doing in conducting the hike that adversely affects the safety or enjoyment of the hikers.
The sweep assists the hike leader in periodically counting hikers to make sure all of them are present.