Aristarchus was a common name, but he was probably the same man from Thessalonica who traveled with Paul to Rome (Acts 19:29; 20:4; 27:2). Acts does not mention Aristarchus was also arrested, so he might have chosen to stay in prison with Paul to support and help him.
Mark the cousin of Barnabas joined Barnabas and Paul on their first missionary trip but left before it ended (Acts 13:13). This caused a disagreement between Paul and Barnabas before their second trip (Acts 15:37–40). Paul and Mark seem to have reconciled at some point (see also 2 Timothy 4:11; Philemon 1:24).
You have already received instructions: There is no other record of this message.