Yes - ants don't harm plants, but they might give you a nip if you disturb a nest in the compost, so break it open and run away for a little while, lol!

Certainly, the ants will in fact be helping the composting process, keeping it well aerated, slowly breaking down the plant material and encouraging beneficial fungi. Having said that ants will not tolerate damp conditions, so they are also a sign that you composter might be a little on the dry side.