Shekeres are a great instrument. The instrument goes by a couple of different spellings: chekere, shekere, agbe and I think calabash in some places.
This instrument has evolved to be the main shaker in rumba nowadays. Early rumba recordings show maracas and nkembi (wrist shakers) being used. Today the shekere seems to be the most common shaker.
Usually you will see one shekere player, though it's not unusual to see two shekere players alternating.
The typical patter is for the shekere to play the downbeats with a pick-up just before the downbeat. For example; a1 a2 a3 a4. I personally like to start with just playing on the 1, and then later adding the 3 and then finally later in a song maybe moving on to playing all 4 downbeats. For some sections it is even fun to play all the 16th notes for a portion of the song to get things cooking, but you really should not over do that, save it for something special.
In my experience rumba shekere is different than that for guiro and bembe, where the shekere takes a more prominent role in the ensemble; creating melodies between three shekeres. However it's not unusual to see guiro shekere patterns adapted to a rumba rhythm, particularly a columbia.
The following are some pictures of shekeres I have made. I really enjoy making the instrument and receive a lot of satisfaction watching people make music with instruments I have made.