Every member can invite 5 people every 10 days. It is a rolling window, not a monthly reset: each invite frees up 10 days after you created it, so if you use all 5 today, the first one comes back in 10 days.
Making one
Go to Invite builders. You get a link to send to the person you have in mind. The page tells you how many you have left and, when you have none, the date the next one arrives.
Things worth knowing
- The clock is the server's, not your computer's. Changing your device's date does nothing.
- Admins have no limit, because they have to be able to fix things.
- You are responsible for who you bring. Invites can be taken away from members who invite people who break the rules.