Today I wanted to put a transaction wrapper around some samples in jmeter. I come from a Loadrunner background so I am used to using lr_start_transaction. To do a similar thing in load jmeter you need to put your samplers under a transaction controller (found under the logic controllers).
The diagram below shows how I put two HTTP requests under a transaction controller.
You can see from the results table for a single iteration of a single thread the transaction wrapper is slightly over the sum of the two HTTP gets by a few milliseconds. Not perfect but a few milliseconds should be OK in most circumstances.
However, I noticed in longer performance tests this can increase. However, you can overcome this by selecting the following option.