Have you noticed that you invite people to apps like LinkedIn, Referral Key and Twitter and they don’t respond? Don’t be despondent, take action. 
Go back to the apps you are trying to connect them to/with and do some serious searching, seriously. There‘s a good chance the guy or gal you want to connect with has more than one email address! I personally have several emails, for a number of good reasons!
I’m not sure why anyone would think that in my business I was not already on all these sites but it seems they do. I regularly get invites and when I respond the systems are asking for me to sign up. There appears to be no way to get them to connect the invite you get with the accounts you already have set up. I’ve tried believe me, more than once.
So, here’s my 2cents worth of advice, search right before you invite. A good place to start is to check other profiles on other apps in case there is already a link there, alternatively, use combinations of personal name, business name and any emails you think may have.
Go for it – J