When a topic is created in either the web or app version, the topic can subsequently be assigned to a user.

Below is an example from the web version where you click on the Assign tab to select a user.

From here you can enter the e-mail address manually or select the user from the list by first selecting the group (Team) to which the user has been invited in advance and then the user himself.

In the app version below, you can assign by either entering the e-mail address manually or by selecting an invited user from the list by clicking on plus (+)