I get guild invitations while playing, what to do?

Hi, sometimes there's invitations that appears on my screen while I play, these ones seems to come from people and they ask me if I want to join tribe "X".

Do these people are real or NPC? What will happen f I accepts? Please help me, I'm new and because I don't know I fear to accept. Thanks.
Don't accept it. Some people just randomly invite anyone they can find because for some reason they want to have plenty of people that don't even bother to interact with one another in a guild.

To stop it you can go to the city and look if there are some social noob friendly guilds advertiving there. You could join one and that would stop other guild invites from appearing and you might get some helpful community of people to chat with and ask about stuff.

If you don't wanna join a guild then press esc, go to the interface options, select social and there you can tick "Block Guild Invites". This will stop all guild invites from appearaning, game will automaticly ignore them for you. If you ever decide you would like to join some guild you would need to go back to the options and untick that.
What Cedrad said.

But don't fear joining a guild. Joining a guild is not a commitment, and nothing happens to you because of it.

Joining a guild does not impose any duties or limits on you, and if you do enter one by accident, or one you don't like, you can leave completely at any time just by typing /gquit in your chat box, or there's a button somewhere in your guild tab to leave. It's all very casual. People come, people go, all the time.

There are serious progression guilds, where you're part of a competitive team, and to be on the team you will have to attend sessions for gearing and raiding, but you don't get invited to a raid team by accident. :) For those, you have to apply and show evidence that you can play well enough.

The guilds inviting you are just basically collections of separate people, who mostly ignore each other, but have a shared extra chat box.
Thanks for your precious answers :)