You can easily assign the created activity to other users by following the below instructions.
Login to Backend
To reassign a activity, open that activity and click on the Re-assign icon in the dialog box. Now select the user in User/Team and click on Save button.
Select the user or team from the look-up records box to which the activity is to be reassigned.
Note: To reassign any activity to any user or team, they should have roles defined, or else it will prompt an error message.
In this way, you can assign the created activity to the other user.