No you wouldn't need to buy invite to create a wallet. You use someone's invite link to create a wallet in Merit. Thereafter, that wallet needs to be activated using an invite token (can be bought or mined). Your inviter will send you a token. Some of us send extra tokens since it is in our best interest that you grow your community/network on Merit. Similarly, others can use your invite link to create their own wallets in Merit. But they will need to be sent an invite token each to activate their wallets as well.

Invite tokens are created and airdropped to wallets (these invite tokens are called mined invites) based on growth score, new addresses, randomness, mining activity. Also, invites can be traded.