The Super Power Club was a "subscribers only" club formed by the staff at Nintendo Power magazine in the mid-1990's. Readers automatically became Super Power Club members if they subscribed to the magazine.
Being a Club member allowed those readers access to bonus material within each Nintendo Power volume. Some of those bonuses included special strategy guides, behind-the-scenes material, contests, coupons for games, and free items such as stickers or collectible trading cards.
Special Nintendo Power volumes
Nintendo Power V44 had an entire section of the magazine specifically for Super Power Club members. It was over 30 pages of bonus material, plus it had a separate 16-page Mario Paint insert.
Nintendo Power V56 had a special "member's only" cover and a 16-page behind-the-scenes special. It also included a Super Nintendo Power Index that listed all SNES game up to that day.