I really don't get the question fully. Are you referring to knowing the people on your contacts who are on messengers? Or do you want to add your contact numbers to your messengers account? I think I'll answer you in relation to the later as it makes more sense of a question.
Adding your contact to your messengers account is quiet simple. It will take just a few steps which starts with you logging into your messengers account on the device.
Secondly, you'll need to go on the settings tab on the messengers account. The last step depends on the type of phone and it's function type.
Anyways, all you have to do is search for sync function in the setting and tap on it to synchronize your phone contacts with messengers contacts. That's it, as simple as that. It's recommended you leave the sync on auto so every new contact added to the phone automatically adds to the messenger account.