If I have both a freelencer and client accounts and there's is another person with the same, a freelancer and a client accounts, he could be a friend or just someone we met on Upwork. If I could be inviting him to the jobs I post and he be inviting me for the jobs he posts, is it against Upwork's terms of service or is it acceptable? Someone please help
You can't hire friends and family. That is not Upwork's purpose.
Yes, Upwork does check for these kinds of things.
Upwork may terminate the accounts of everybody involved if they catch you doing this.