this.betTable$: BehavorSubject<Table[]>;

getTable(): Observable<Table[]> {
    return
    ... fetch from API
    .mergeMap(betting => {
        this.betTable$.next(betting);
        return this.betTable$;
    }
}

join() {
    this.service.join()
        .mergeMap( () => this.service.getTable()
        .first()
        .subscribe()
}

remove() {
    this.service.remove()
        .mergeMap( () => this.service.getTable()
        .first()
        .subscribe()
}

...
this.service.getTable()
    .subscribe( () => {
       // some processing to format row to display
       this.data.push(row);
    }

betters$: BehaviorSubject<Table[]>[];

this.betTable$: BehavorSubject<Table[]>;
this.betters$ = [];

getTable(round) {
    let key = 'table:round:' + round;
    return
    ... fetch from API
    .mergeMap(betting => {
        if (key in this.betters$) {
            this.betters$[key].next(betting);
            return this.betters$[key];
        }

        // add new observable to array
        this.betters$[key] = this.betTable$;
        this.betTable$.next(betting);
        return this.betTable$;
    }
}