How do I write sensible FOREIGN KEY CONSTRAINTS if the PRIMARY KEYS from two tables can reference multiple of the other table's PK

I am setting up a database for a university and I'm struggling to setup sensible FOREIGN KEY CONSTRAINTS (and even tables) because I have two tables in which the PRIMARY KEYS of each can relate to multiple PRIMARY KEYS of the other table and vice versa: Concrete example: - I have a "users" table of all staff members where the primary ID is AUTO_INCREMENT - I have a "workgroups" table of all workgroups in the university where the primary ID is AUTO_INCREMENT - in the "users" table, there is a column called "workgroup(s)" which uses workgroups.id as a FOREIGN KEY - in the "workgroup" table, there is a column called "leader(s)" which uses users.id as a FOREIGN KEY my problem is that in rare cases, a person/user can be the leader of multiple workgroups AND in even rarer cases, a workgroup can have two leaders. I am reluctant to give the "users" table several "workgroup" columns (like "workgroup1", "workgroup2", etc) and equally reluctant to give the "workgroups" table several "leader" columns. And I believe there is no way to assign an array of user.ids to the workgroups.leader(s) column and have that be a FOREIGN KEY which actually references multiple user.ids, right? So, if users with IDs 10 and 15 are leaders of workgroup "genetics" (workgroups.id = 1), I cannot assign "workgroups.leaders @ id=1" to be [10,15] and have that actually be a FOREIGN KEY which references two user.ids correctly, right? I also have setup FOREIGN KEY CONSTRAINTS between the two columns "workgroups.leaders" and "users.id" in both directions going "ON DELETE SET NULL" and "ON UPDATE CASCADE". The intention being that if a user who is a workgroup leader is deleted, the workgroup should have NULL leaders until manually assigned some other user as leader and secondly: If a leader's user.id or a workgroups's id changes (both of which shouldn't happen), it would be updated in the other table, respectively. I assume that I have to setup these many-to-many relationships between "users.id" and "workgroup.leader(s)" in a third table?? So take out "leaders" column from "workgroups" table and take out "workgroup" column from "users" table? And then put them in a third table where a user can be linked to be leader of multiple workgroups AND have the FOREIGN KEYS set in this third table? Should I do this for all user workgroup relationships in a forth table? Because, aside from being leader, any user may be a MEMBER of several workgroups (without being its leader)... or could I put this all in a single third table with 4 columns like "relationship_id [INT, PRIMARY], user_id [REFERENCES users.id], workgroup_id [REFERENCES workgroups.id], is_leader [BOOL]"? Do I even need "relationship_id" here if this table is only used for joining on the two FOREIGN KEYS? I just hope that it'll never happen that a leader of one group can be in second group without being the second group's leader...