To give a user access to a door it requires 5 steps:
Ensure you are an Access Admin (Allows you to see the Access portion of Command)
Make sure you are a Site Access Admin for the site containing the door (Allows you to see the access controllers and doors at a site)
Assign the door to an Access Level
Associate the Access Level to the Access Group
Assign the user to an Access Group
The steps are detailed below.
Ensure you are an Access Admin
If you add an access controller to your org you are automatically made an Access Admin. Otherwise another Access Admin needs to go to Admin > Access Control > Access Users to elevate your permissions
From the Access Users page, Check the box by the user to promote to Access Admin, select Assign Roles, then select Access Admin
Ensure you are a Site Access Admin
This will allow you to see the access controller and door on your Home and Devices pages. If you are an Org Admin, you can give yourself Site Access Admin for any site. If you are not an Org Admin, then a current Site Access Admin for the site with the door in question will need to make you a Site Access Admin.
To do so, they will go to Admin > Access Control > Access Sites
Select the Admins button for the site to grant Site Access Admin to, then select the + Add... button in the top right
Find the user in the list, check their name and select Done to make them a Site Access Admin
Assigning a door to an Access Level
Navigate to the door's page and select Access Levels
From here select Add Access Levels, to see a list of all unapplied Access Levels to this door. Check the box for the Access Level to add this door to and select Add
Assigning the Access Level to the Access Group
Navigate to Admin > Access Control > Access Groups
Select an Access Group, select Access Levels and +Add Access Level
Check the box to the Access Level the door was added to then select Add
Assigning a user to the Access Group
From Admin > Access Control > Access Groups, select the group to add a user to. On the right side of the page select +Add...
Check the box next to the users you want to add to the group and finish it by selecting Add
Upon completion of the above steps, the user will be able to unlock the door.