I had people tell me usually refers to something that was said to you. For example: A: How did you know that the test results are already out? B: I had people tell me, in case I missed it. I had friends join me usually means that your friends joined you for something. For example, A: Did you go to the party alone? B: No, I had friends join me. I didn't want to feel out of place. The first phrase refers to you asking someone to tell you something. The second phrase refers to you asking someone to join you to do something. :)

'I had people tell me this' is a sentence in the past simple which has the same meaning as 'People told me this' or the passive form 'I was told this by people' 'I had friends join me' is the same. It's a past simple sentence, meaning the same as 'Friends joined me' or the passive form 'I was joined by friends'. The construction is like this: Subject + ['have' in the appropriate form] + object + main verb in infinitive. The sentences you suggest don't work, I'm afraid. As you can see, it's the verb 'have' which you need to conjugate, not the verb 'tell' or 'join'. Those stay in the infinitive form. If you want to put the sentences into the present perfect, they would be like this: I have had friends tell me this. I have had friends join me. or, for example, in the present continuous: I'm having friends tell me this. I'm having friends join me. That's how the construction works. Whether or not you actually NEED to use this is another question entirely. Unless your English is very advanced, or you are being forced by your teacher to study every verb construction that exists, I wouldn't worry about this use of 'have'. It really isn't essential, and there is no situation in English where you'd be obliged to use this form. There is always a simpler alternative.