Yes, and you don't even need to make your changes available on GitHub. The GNU Project's attitude to selling free software is summarised in this piece, and they are very much in favour of it.
You do, however, need to convey a copy of the complete corresponding source code (CCS) to each of your customers (ie, those to whom you convey the product), as per AGPLv3 ss 4-6. In addition, should you run your own instance, you will need to convey CCS to anyone who interacts with your instance over the internet and who subsequently requests such a copy (AGPLv3 s13).
In practice, keeping all development in public, on some site like GitHub, will be a good way to fulfil these requests - but if GitHub folds, your obligations don't fold with it. There are some other obligations of which you should be mindful, including but not limited to the requirement to preserve existing copyright notices; you should review the AGPL with your lawyer to avoid any oversights.
And if I were going to do this, I'd take care to warn my customers that they, too, will have s13 obligations to the users of their instances.