You finally got that invite to the super secret PoGo server you've been wanting to join, yes!
So... why isn't it working? There are a few reasons your server link may be 'Invalid'.
What to Check
Expiration: Your invite link may have expired. In this case, please contact the server owner and ask them for a new invite.
Invalid Code: You may not have a legitimate invite code. Please check your code and try again. Remember that invite codes -are- case sensitive!
Ban: You may have been banned from joining this server. Bans are based on IP address, so you would need to contact the server admins and ask them to revoke all bans in their Server Settings. Just because they remove your name, does not mean the invite will work.
100 Server Limit: A user cannot be a member of more than 100 servers. If you are in too many servers, you'll need to leave one before you can join.