How to write research results when some study participants dropped out? The research (Delphi study) started with NN study participants in Round 1 (gathering demographics). Ten percent of the participants dropped out and did not complete Round 2 or (of course) Round 3.

I'm unsure how to report this. 

Do I report the demographic results and then add a note to the Round 2 and Round 3 results ("Note: Of the NN people who completed Round 1, only Nn completed Rounds 2 and 3.") 

Or do I remove from the Round 1 results the data gathered from the drop-outs?

Or ... ?
It is a very bad thing to remove people who dropped out from your data set.  The problem is that you do not know whether dropping out is correlated with the effect that you are studying.

For an extreme example, consider a study on the effect of being shot at on soccer ability.  In round 1, people play soccer, then they get shot at randomly with a gun that might or might not hit them, then they play round 2 of soccer, then they get shot at again, and then they play round 3 of soccer.  Of course, anybody who actually gets hit when they are shot will probably drop out.  If you eliminate those people, you will vastly underestimate how badly soccer players are affected by people shooting at them.

This may seem like a rather extreme example, but things very much like it happen frequently in biomedical or psychological studies, just with less obvious causal connections.

Report exactly what happened, and take the missing people into account when you are computing your effect size.  If you need help on the technical aspects of that, you should ask on Cross-Validated.SE.