If you host your own instance or are on a dedicated plan, you can do this in several ways:
- Automating via Workflows a welcome message in a direct or group chat, or a private message.
- Automating a message in a topic where you send welcomes, also using Workflows.
- Filtering categories with specific custom groups and permissions for “unlocking” access as the user progresses and introduces themselves through badges and some queries, to then be welcomed by staff or existing users.
Discourse includes discobot in its core; although it is outdated, it can work well for simple, neutral welcomes. The text of the messages sent is translatable, and you can configure them from your admin panel.