When “X has a Y,” there should be a column in X’s table for Y, then create a relation column from X to Y. For example, replace X with Member and Y with Church.
When “Y has Xs” or “Y has many Xs,” again there should be a column in X’s table for Y, but you instead create a multiple relation column from Y to X. For example, Y is Church and X is Group.
Have you watched the videos on relations yet? The long video on creating a Conference app would also be very good to watch for this.
Use a form button, in the form panel, a choice component containing all available groups. When the user then submit the form, a new row will be added in a sheet with at least 2 columns : group, participant. Each row of the sheet will be used to determine who is part of each group.
Then you can play with formulas to know if a given member is member of a given group, or determine to which groups a member subscribes. And you will be able to display what you want user per user data.