There are two main usages of the term Ritual in this context.  One is the repetition of activity that typically becomes unconscious after a sufficient number of times; that is, a habit such as always setting up the chairs for the meeting in a circle.  The second usage is a special time set aside with explicit intention where the group engages with modes beyond the rational in pursuit of a desired result.  These uses have in common the demarcation of time and/or space in the invocation of the activity.  Rituals are often used at Openings, Closings, to shift from one modality to another, or to deepen or bring in a different energy.

When a newcomer (or outside consultant) joins with an existing group, the culture will include tacit rituals as well as more formal rituals that the newcomer needs to learn in order to mesh well.  Examples of tacit rituals include turning off cellphones when the session starts, dressing in a particular style of clothing, or going out to the pub together after the meeting. 

Even a tacit ritual may have significant symbolic meaning.  For example, setting up chairs in a circle implies that all participants are equal and that anyone may speak, as contrasted with rows facing a podium.  Having some things that are consistently done the same way saves time and energy of having to figure it out from scratch each time, and builds a shared sense of group culture and identity.

More explicit rituals often include an altar, which may carry items with meanings and connotations for the participants.  Ritual is a way of honouring that we are much more than our minds: our spirits, hearts, and bodies are all brought into it.  People may speak, dance, sing, have silent times together, or more. 

Groups vary widely in their use of non-rational modes.  For example, some religious groups such as the Salvation Army or Quakers might pray together before making a decision, while most secular groups would be acutely uncomfortable at such a suggestion.  Meanwhile an activist group might start with a round of personal check-ins on how people's lives are going aside from any business at hand before starting the meeting, which wouldn't fit at all in many businesses.
 
Cautions & Caveats:
Rituals can become impediments to creativity, to change agility, to welcoming newcomers and new rituals; they can fuel resistance to change.