I’m a member of a very well-moderated subreddit. Part of what the moderators have done to make sure both that outsiders feel welcome and that people have adequately understood the directives of the group, is they have set up an automated comment that appears as the first comment on every new entry, saying something like “Welcome to —-! This is an automated response. Please take a minute to review our community guidelines here, here is a useful article on what is not allowed and why we don’t accept ——- questions... Please familiarize yourself before commenting”. It serves the dual purpose of making sure that everyone gets at least ONE comment (that may sound silly, but it can do a lot for people who are timid about public posting) and greatly increase the odds that any OP actually reads the guidelines. Personally, I always find SE community guidelines and other resources difficult to locate, and it’s useful also because it means everyone can easily locate guidelines from any question anywhere on the SE. It sounds like it would junk things up a lot, but it really doesn’t take up much space. Please consider.
We don't have any control over automating a welcome message, however, posts from new users appear on the post review queue.
So a moderator or one of the community members reviewing new posts by new users usually posts a welcome message, unfortunately a lot of the time that also includes flagging a post for deletion and then the moderators have to think whether or not to let a post slide because it's from a new user.
By the time it gets to the flag queue and a moderator sees the message, unless we look at every one of your posts (and if there's a flurry of them it is hard) we don't know if it's your first post or not.
So, I'd heartily encourage those reviewing and flagging posts by new users to post a welcome message even if they're flagging for deletion. At least that way the moderator could second the flag reason and in choosing whether to delete the post supplement the welcome message.