I would like to be able to make an API call in Bridge to generate a list of all groups/groupIDs that a user is a member of. To my knowledge, this can only be done for me as the "current user" at this time.
Unfortunately there isn't an easy way to do this over a single endpoint. As you've seen, getting a user doesn't provide the groups. Getting a group provides the members, but it's not a filterable/searchable list.
I'm assuming you want to sweep down a list of users and find this for a set of people?
Without using Javascript, you could do this natively in Bridge like below. Warning though, it will be slow.
PARENT WORKFLOW
CHILD WORKFLOW 1
CHILD WORKFLOW 2
JAVASCRIPT
BTW, to see how to write Javascript to return a formatted array to do Add Row - check the post here. You may or may not need the "chunking" function described in this, and don't need the Resource Management pieces, but you'll see in the Javascript code that there's a section that takes the data and formats it into an array of row objects to pass along to Add Rows. Your row objects, instead of using toTop: true as the row location specifier, should use the {"parentId": xxxxx, "toBottom": true} format to insert the rows underneath the row that has your user listed in a sheet. You'll need the rowId for parentId, that comes from your user data.