There are many different ways to grow a channel or chat - write good posts, buy ads in related channels or chats, buy ads in other social networks in thematic groups.
There are other ways - such as sending private messages with the correct text and a link to a channel or group. If you have a group, then you can add members of a similar group there and there is a high probability that they will stay. There are a lot of options. But you need to understand which ones are acceptable to you, what niche and what budget you have for the promotion and growth of your project.