Rather than uploading all of your users you can get them to self-register. When you get users to self-register they complete all of the user information themselves including user fields. 

How do I set up self-registration?

You have to have Admin permissions for this. Navigate to Admin then 'People' in the sidebar and then 'Self-registration'. Here you can create a form, when you create the form you will then have a URL that you can send users to where they will set themselves up. 

Creating and sending a self-registration form

1. Give the form a name
2. If you want to assign the user that registers with the form URL to a group you can select the group
3. Add any of your user fields that you would like for the user to complete when they are registering
   1. If you are using the form for one value on the user field you can force the user field to set that value if you wish. For example, you may have a user field call location, and you may be sending this self-registration link just to the users that are in Germany, in which case you could force the user field you named location to have the value Germany for these users. 
4. Save!
5. You can now go back into the form by pressing on the name in the list and you will find a unique URL. Copy and paste this to any communications you may be using so that the recipients can register. 

Users will then be presented with your form when they follow the link

Tip! - If you are using SSO you don't need to do the above make sure you follow this guide and turn on the 'Create platform user on authentication' toggle