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There are a couple of problems with this code:
1. Getting all users emails is very inefficient. What if there are 100,000 users? This line will load **all users**, build a model object for every record, just to get a single email value... This will eat up server memory very fast. The same inefficient way is used to check existing invitations.
2. You're building emails list once, checking the size, then building it again, checking size again. Nesting "ifs" like this is bad coding style. You should build a list and then process it, once.
3. You're using deprecated #find(:all) method, this is a Rails 4 app man.
4. You don't respect existing coding style.