So, as a missionary, invitations are super common, and my invitation is simple: just let the missionaries know (maybe once a week) that you want to help them. Then, tell them what days you are available. Trust me, they will be soooo grateful for your willingness to help out. Even if you are only available once a week or even once a month, let them know! Regardless of if they need you that particular night, they will be happy to have your support!