Should a column referencing a group member have its foreign key directly point to the user or add an id to M2M table which represents users in a group

Question

I'm creating a many-to-many table in a database which contains a column referencing members of a group as well as a column referencing an event.

I'm questioning whether I should set the column referencing the group member to reference the main user entry in the users table or instead add a primary key to a groups_users many-to-many table and reference the user_id there so that when the user is removed from the group then they are automatically removed from the event with ON DELETE CASCADE instead of having to manually remove them with a seperate query.

From what I know, the pros and cons for directly referencing the user are as such:

Pros:

Less data used up by an auto incrementing id for members of groups

Cons:

Have to manually remove users from events with a seperate query

The pros and cons for the other approach are a reverse of the first approach.

users table example:

id	username
1001	User 1
1002	User 2

groups table example:

id	name	owner_id
2001	Group 1	1001
2002	Group 2	1001

groups_users table example:

id (only if referencing members of group rather than users directly)	user_id	group_id
3001	1001	2001
3002	1001	2002
3003	1002	2001

events table example:

id	group_id	name
4001	2001	Event 1
4002	2002	Event 2

events_users table example:

user_id / group_member_id	event_id
1002 / 3003	4001
1001 / 3001	4002

Answer 1

First, I would not add an artificial primary key id to the groups_users table. The natural key consisting of (group_id, user_id) is perfectly fine as primary key.

Concerning the question whether events_users should have a foreign key to users or to groups_users, ask yourself the question: is it the group membership of a user or the user itself that is related to an event? For example, if I am in two groups, could I be related to an event twice? The answer to these questions will tell you how to model this. From my point of view, a the foreign key should be to the user, but then you know the situation better.