Hello ​@wealthywithchrisdl ,
For a contact to become a subscriber in your Manychat account, you need to create a comment reply automation.
If someone leaves a comment but there is no active automation, Manychat will not take any action.

Below is a help article from Manychat that explains step by step how to create this automation. When you click the button to create a new automation, you can also choose from Manychat's pre-made templates if you prefer not to start from scratch:
https://help.manychat.com/hc/en-us/articles/14281316989724-Instagram-Post-and-Reel-Comments-Trigger