You can infer responsibility for follow-up actions to a user by assigning the observation to him/her. Here's how you do it:
When reporting an observation, you may assign it to one or several users. Before clicking 'Send Observation,' select 'Follow-up actions needed.'
Note that some forms are set to always require follow-up actions, thus not providing you with the option of selecting 'No follow-up actions needed'.
From the 'Assignees' -field you may select one or several assignees from the list of eligible users. You may also set a deadline.
You may also set a deadline.
When all assignees have been selected, hit 'Send Observation.' The assignees will receive a notification of the observation, and they are visible in the observation page;
as well as the Observations list;
You may add/remove the assignees, change the deadline, or even withdraw the need for follow-up actions by clicking 'Edit' underneath the downward pointing arrow in the 'Assigned to' field.