In the fediverse, we are all the welcoming committee! If someone you know from elsewhere joins, make sure you interact with them, helping them feel welcome and included.
* Reply to their posts, @mentioning people who might be interested in the topic.
* Boost their best posts to help people find and follow them.
* Subscribe to their feed to get a notification every time they post, so you remember.
* encourage them to post an #introduction, to help fun folks find them.