When inviting a specific player to co-op, you need to enter their username exactly (case-sensitive).
If you don't enter a name, it will just look for any player with a suitable army and they might cancel any request.
The player who joins you must have a similar strength army - 70% weaker to 30% stronger than your army - ideally in the same area.
If you are doing all this and it's still not working, then please let us know and if you could attach your log1.txt file (and the log1.txt file of the person you invited) that would help us figure out what is going wrong.
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