A substitute is a player invited to a specific group's substitutes pool. Subs are not regular members -- they don't take a member seat and they don't auto-fill every recurring match. They get pulled in match-by-match when you need them.
Each group has a dedicated Subs page (below the Members list). Owners and admins invite subs by sending them an invite link. Once accepted, the player appears in the group's sub pool and can be invited to individual matches.
Open a match and tap the Invite Subs action. Pick one or more subs from the pool to invite. Each invited sub gets a push notification with a deep link to the match. They can confirm In or decline.
When a regular drops out, subs already invited to the match can take the open spot. A confirmed sub fills a real roster slot for that match -- they receive the same notifications and get access to the match chat.
If a regular member opts back In after a sub has filled their spot, the sub is automatically bumped. The sub gets a notification explaining what happened, and the admin is notified too. This keeps regulars prioritized without losing track of the sub.
A sub who can no longer make it can opt out from the match. They are removed from that match's roster and chat. The admin is notified so they can invite another sub if needed.
The same player can be a regular member in one group and a sub in another -- the two roles are independent. Roles are scoped to each group.
Group owners and admins can remove a sub from the pool at any time from the Subs page. This removes them from any future match invites but does not retroactively remove them from matches they've already played.