I think you should initiate a conversation and make some pretty good discussion there.
For example : a new telegram user have been added in the group, that means if they are asking for questions, you should reply with a pretty good answer or explain the technical details or the purpose of the project to the one who want to be enlighted by your good answer.
There is no rule that you should add more members but it is good that everytime that there is a new member then promote the project using good words, that can spread awareness about the project and the team as a whole.