All you have to do to participate in the challenges, is:

If you are from another BOINC team:

make sure your team founder(s) add your team to the challenge on boincstats also. The more, the merrier.

These are special challenges (initiated by team Gridcoin itself):

Our team Gridcoin will also participate in following challenges:

Nov 8-12: CRUNCHATHLON PrimeGrid [MONTHLY RUN - RACE XIX] (confirmation needed)
Nov 11-13: 19th FORMULA BOINC sprint challenge
Nov 15-22: yoyo@home challenge (confirmation needed)
Nov 16-20: CRUNCHATHLON GPUGRID [MONTHLY RUN - RACE XXXVII] (confirmation needed)
Nov 17-20: PrimeGrid's: "Pierre de Fermat's birthday challenge"
Nov 23-26: 20th and last FORMULA BOINC sprint challenge for 2017
Nov 26-30: CRUNCHATHLON WCG [MONTHLY RUN - RACE XXXI] (confirmation needed)

Chronological, all:

Nov 8-12: CRUNCHATHLON PrimeGrid [MONTHLY RUN - RACE XIX] (confirmation needed)
Nov 11-13: 19th FORMULA BOINC sprint challenge
Nov 15-22: yoyo@home challenge (confirmation needed)
Nov 16-20: CRUNCHATHLON GPUGRID [MONTHLY RUN - RACE XXXVII] (confirmation needed)
Nov 17-20: PrimeGrid's: "Pierre de Fermat's birthday challenge"
Nov 23-26: 20th and last FORMULA BOINC sprint challenge for 2017
Nov 26-30: CRUNCHATHLON WCG [MONTHLY RUN - RACE XXXI] (confirmation needed)

Past challenges:

Erkan (find me on IRC and twitter)