If your Jira Admin granted you rights to use the Werewolf for Jira app, you can find the relevant section in your profile menu.
Note that all your actions on behalf of another user are logged. The audit log is available for your Jira Admin.
Navigate to Werewolf settings, select a user and push Login – now you are able to perform the actions in the system on behalf of the user you selected.
You won't be able to find any user in the list of options – only those who have been included to specific groups (specified within the Werewolf settings).
You can also navigate to a user and turn into them by selecting the Werewolf option in the More drop-down menu.
You may even transform into a user without Jira access. In this case, you'll see the Jira as if he/she would have access to it, you won't be able to create an issue though.
This may be useful if you are going to grant a user the access and are not sure about information he/she will get access.
Don't forget to 'un-werewolf' when finished – just navigate to the profile picture and select the Un-Werewolf option in the drop-down menu.