Step 1 - Go to the activity card and add members in the roadmap from the bottom corner of the respective activity card. You will be able to assign activities to members who are already added to the project. In case a member is not added, add them to the project first.
Image: Assigning members from roadmap screen
Step 2 - Once the member is assigned, they will be responsible for the activity review cycle. They can change the status of the activity and mark it for review. Multiple members can be assigned to an activity.
Image: Inside activity, view for an assigned member
Step 3 - The owner can choose to unassign any member as and when needed
Image: Edit activity modal window inside activity from where, the owner can change the assignee