I hate when this happens, usually I just wait for it to come back and that can range from a few minutes to a day or so. My best advice for inviting someone is through a party or through your console friends list(unless that's down then your out of luck) once they are in your fire team there is a good chance your whole list might come back. That's just from my personal experience
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This has been happening for two weeks now. No luck of it coming back. :/