A welcome post is the first thing members see when they open the app, so it is worth writing before you invite anybody. This is about what to put in it — for the composer itself, see Create a post.
Pick the right audience
A new post offers up to three types, and which ones you see depends on where you are:
Team post — the group you are currently in. Always available.
Multi group — several groups at once. Offered only when the group you are in has subgroups you are allowed to list.
Club lobby — everyone in the organization, including people who are not in any group yet. Offered only at the organization level.
For a welcome post, Club lobby is usually the one you want: the club lobby is where a new member lands before they belong to any group, so it is the only audience that reaches somebody on their first day.
What to put in it
Who you are and what the app is for at your club — members arrive without context.
What you will use it for: schedules, payments, team news. Set the expectation that this is where things get announced.
What they should do first — check the calendar, complete a registration, connect a child.
Who to ask when something does not work, and how.
Posting sends a notification to everyone in the audience you picked, so write it once and get it right rather than posting three corrections.