Edit: now there is yet an easier way to do this - when creating your group, just mention the full bot name (eg. @UniversalAgent1Bot) and it will list it as you type. Then you can just tap on it to add it.
Old answer:
- Create a new group from the menu. Don't add any bots yet
- Find the bot (for instance you can go to Contacts and search for it)
- Tap to open
- Tap the bot name on the top bar. Your page becomes like this:
- Now, tap the triple ... and you will get the Add to Group button:
- Now select your group and add the bot - and confirm the addition