Whoever sends the invites will be the leader of the Convoy, and the only person who can start events. This will bring up the Xbox gaming bar, and players can choose who to invite from their Microsoft friend list. To keep it between friends, choose 'Invite Only'.
To send a Convoy invite to friends, bring up the pause menu, and then press Y on Xbox consoles, and select 'Create a Convoy'. So, invites will appear as a notification, and players can hold down the middle Xbox button on consoles to accept the Convoy invite, or open the Game Bar on PC and accept the invite. This uses the Xbox/Microsoft friend system, even on Steam. However, PR stunts like Drift Zones in Forza Horizon 5 must be completed solo. After completing the opening showcase and the next two events, players can invite friends to their Convoy, or be invited to one.
This is basically Forza's version of creating a party. To participate in Events, map exploration, and other multiplayer modes together, friends must first be a part of the same Convoy.