If you have made your decision to add more and more members to your public channel, you have this power to make it possible other users join it. In this action, many members can join simultaneously and number of members can be raised all at once.
To do so, tap the channel name at top of the page and see channel Settings.
An option shown by “Username” is clear and by copying the channel username as a message beginning with @ in a private channel of yours and then sending that as an Invitation Link. Every one of members in your private channel can join by touching this link and then be added to your public channel.