Newbie data modeling help needed

I'm trying to create a Secret Santa application. The basic entities
are users, groups, relationships, and suggestions. (If User1 has a
relationship with User2, then User1 will not be assigned to be Secret
Santa for User2).

I'm trying to figure out how to model this so that I can leverage
Rails.

The users table contains: id, full_name, email, username, password
The groups table contains: id, group_name, group_password
The relationships table contains: user1id, user2id, group_id
The suggestions table contains: user_id, group_id, suggestion
The users_groups table contais: user_id, group_id, assigned_pick

I'm a bit confused as how to model the relationship between the users,
relationships, and groups table.

Also, the group_id is a foreign key in almost every table. Am I missing
something here?

Any feedback is greatly appreciated!