Hello there
Did you check if your inventoy isn't full?
If you're at 50/50 items, the join button isn't available anymore
If it still doesn't work, are you sure you unlocked the dungeon you're willing to join? Or are you sure you have the required level?
If these ideas don't fix it, please email the support there : http://support.spacetimestudios.com/
They will help you
Cya and good luck ^^
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