Depending on the type of information you can define at the beginning what it will be and the receiving server can read it. Thus allowing information to be displayed accurately in real time with about a 1 second delay.
You would need to send this information via PublishAsync, whenever a player is added to the party, along with a sort of PartyId, so that the game wouldn’t confuse other parties with yours.
This is similar to how developers form server IDs for their games to create join able servers like in the old game Apocalypse Rising. Although the approach then was different and for a different reason.
int PlayerCount; // Just a count of the players in the party.
int[] Players; // An int array of the party member's UserIds.
string PartyId; // A random GUID.
string CustomMatchId; // The ReservedServerId.
int LeaderId; // UserId of the party leader