Where to find user id

Hi

I would like to use our user id as sort of invitation code to new registration, therefore I need to have a list of user id of all my users. Saw that UM does not make randomized id so I have installed the Random ID plugin as well. Still, I am not sure where I could look for user id.
Regarding to the above thought, I am trying to insert individual user id to Account Welcome Email so that all users may receive unique code (i.e. their user id) in their email. Is there any way to achieve this?
Last but not least I wish to export all user details to e.g. excel for record. Could you kindly suggest how to do it?

Thanks and have a nice day!