I've seen many groups do "Cross Promotion". So you promote them in your group, and in turn, they will promote you in their group. You could also advertise your group in other TG groups such as The Coin Farm (although you'd might want to check that it's cool with the moderators to avoid getting kicked from the group).
That's great advice. In the end though, content is king. Maybe having a medium / steemit account might elp with promotion as well, you can post your annouuncements on your telegram channel and link to a steemit account for further details. As long as our content is quality, you will gradually succeed.