So if I join the test net, will I get to keep my test net balance once it launches?
Yes, you can read it on the first page of a site, on the bottom:
"Join the Idena testnet now to get the advantage of early mining or subscribe to be notified about the Idena mainnet launch. The Idena mainnet launch is planned for Q3 2020. All the testnet balances will be kept in the mainnet."