Assign to multiple accounts
Click on the Users tab.
After you've added users, if you want to assign them to other accounts, select those you want to edit and then click on assign to accounts.
When the modal displays, select the accounts and the role you want to assign these users as.
Add me as a user
Note: this is only available for account admins.
You can go over your list of accounts and add yourself to each account by clicking on Add me as user.
Alternatively, check the accounts you'd like to add yourself as a user and then from the bottom of the table select Add me as user.