Possessed in common.
Owned by the members.
Having the same relationship, each to each other.
Collective, done or held in common.
Reciprocal.
A mutual organization.
Either of a pair of people who follow each other's social media accounts.
A mutual fund.
Relating to relations.
Friendly and peaceful.
Dealing with the whole of human relations and their social context, rather than an independent and private space.
Of a database technology using tables and the principles set forth by Dr. Edgar F. Codd. (Contrary to popular notion, "relation" in this context refers to tables, and not linkages.)