(MySQL) SET column with values from a foreign key constrant
Schematically.

CREATE TABLE users ( id PRIMARY KEY, ... );
CREATE TABLE groups ( leader_id REFERENCES users (id),
                      member_id REFERENCES users (id) );


The keys (including primary) and constraints in groups are dependent by:


does the user may be a member of one group only, or of a lot of groups;
does the group leader may be a member of another group (rings possible);
does a leader must (or may) be a member of his own group.