I have the following pandas dataframe containing sports match information:

```
tournament    playerA    playerB
1             A          B 
1             C          B
1             D          A
2             A          B
2             A          C
2             A          D

```

I now want to add a new column (or create a new dataframe; whatever is easier to accomplish) containing information on how many players participated *per* tournament. *Players participated* is the set (no duplicates) of *playerA* and *playerB* columns.

If a new column is added, the new dataframe should look like this (tournament 1 has 4 players participating, i.e., *A, B, C, and D*; tournament two has 3 players participating, i.e., *A, B, and C*):

```
tournament    playerA    playerB    players_sum
1             A          B          4
1             C          B          4
1             D          A          4
2             A          B          3
2             A          C          3

```

If it's easier to create a new dataframe it could/should look like this:

```
tournament    players_sum
1             4
2             3

```

What I tried until now: I tried a `groupby()` and then `apply()` or `transform()` however I dont know how to merge the information of two columns (*playerA* and *playerB*).

Thanks, your help is highly appreciated.