Users are the team (internal) users who are added to a plan to manage it on behalf of your team. There are 2 types of user roles on a plan:
Note: The exception to the above is that users with the team manager role can access any plan with permissions equivalent to the owner user. I.e. They can work on any plan, add/remove users, etc. even if they're not a member of it.
When a plan is manually created, if the Make me the plan owner switch is on (the default), the user creating the plan will be assigned as the owner and sole member of the plan. The user configuration on the template is ignored.
If Make me the plan owner is off, or when plans are created from invite links, the plan will have owner and member users as defined in the template.
First open the plan you want to add a user to. Then click the Person button at the top right of the page:

You'll see the people page, which is where you can manage the people (participants and users) invited into the plan:

Click Add a user, then select a new user to add to the plan:

Click Add user to save the change. The user will receive a notification.
In the same place (if you are the plan owner user or a team manager), you can Remove any user. You can also use the Make owner button to transfer the plan owner role to another user.
There's also options on the plan list to replace the plan owner on one or many plans:

This option will add the selected user to the plan (if needed) and make them the owner. The previous owner user will be demoted to a member user but remain on the plan.
Tip: You can also do this in bulk by selecting multiple plans and using the Change owner bulk action.