This is so easy to add someone to a telegram group; only you have to follow the instructions given next and note that you must be the admin of this group to add anyone.
1. Open the telegram screen. 2. Select your group. 3. Tap on the group name. 4. Now, as you can see, add members with a plus icon. 5. Tap on it and select the contacts you want to add to the group, then click on the right button right below.
Apart from this, you can send a group via the link at the first position in the contact list to join your group for anyone.