You can easily add multiple users to the plan using multi-select with search functionality. This will allow you to search by user name to quickly find and add your team members.
If you enter an empty plan a message will show up with the Add user button.
Click the button and a modal will show up.
Here you can either start typing to search users by name or click the input to open up the user's list.
You can select multiple users to easily add your whole team at the same time.
If you enter a plan with users already present then there is another Add user button under the user's lists.
After you click it the same modal that allows you to search for users will show up.
Other
If you don’t see any results or an error shows up during the search make sure you have sufficient permissions.
Browse users and groups global permission is required. You can set it on the following page:
https://{YOUR_INSTANCE}.atlassian.net/secure/admin/GlobalPermissions!default.jspa