Invitation credits are credits that can be used to invite new members or at least reduce their waiting time.
You can check how many credits you currently have and how many you recently earned on the Credits tab of the Invitation page.
You earn credits over time simply by being a member and by playing ranked games. You receive a certain amount of credits for every day and a certain amount per game. Parameters are set in a way you should almost be able to invite a full member every year.
When refreshing to collect new credits, the earned credits for time being a member are rounded down to the cents! For example, if you receive 0.12 credits/day (1 every 2 hours) and you collect after 3 hours and 55 minutes you will only receive 0.01 credits and the remaining 1 hour and 55 minutes is discarded! If you had waited another 5 minutes you would have collected 0.02 credits. If you collected no credits at all (because you refresh within 2 hours), your time will not reset and nothing is lost.
There is a limit to the amount of credits you can keep. It is currently set to 100, the same amount needed to fully invite a new member. Any credits you earn over the limit are discarded. You can only collect time credits back up to a month, any time from before a month ago is discarded. Although credits can be earned by the cents (0.xx credits), they are rounded when spending them.
When inviting a new member you select the amount of credits you want to use. The minimum amount to use is 10 (or less if less are needed for a full invite). The maximum amount to use is 100, how many you have or how many are needed for a full invite. You can select an amount in steps of 10 (rounded) credits.