There is nothing wrong with offering non-paid work to juniors, just expect the work to require polishing and ensure you return the favour with training.
The key to successful copywriting is fulfilling an informational desire and publishing into relevant spaces. However there are some important rules to follow to help ensure that content is consumed.
With so much chatter online, even the best content can require a little nudge. Seeding and content amplification is recommended to help it gain traction and drive consumption.
Only content produced with your audience in mind will see that content consumed and shared. What are their informational desires? Fulfill them. Become the credible source