You need to use the user search API with an elastic search query string.

Here's a curl example:

API_KEY=.... GROUP_ID=f4a64b10-1cf6-4289-84c0-c3035f3b78bc curl -H "Authorization: $API_KEY" 'http://localhost:9011/api/user/search/?queryString=memberships.groupId:'$GROUP_ID

If you are using the database search engine, you cannot find all the members of a group.

It depends on the issue, but there are two things you can do. Note that FusionAuth typically sends the query you post to the /api/user/search endpoint with the query parameter straight through to Elasticsearch

First, ensure you are running elasticsearch and not the database search engine.

Then, for some queries you can see the generate ElasticSearch query strings by clicking on the 'advanced' option in the user search area of the admin UI. This can be helpful.

Finally, try running the query directly against elasticsearch and seeing if it works. Here are examples which will pull back all the data in your elasticsearch cluster (beware!).

curl -XPOST -H 'Content-type: application/json' "http://localhost:9021/_search" -d' { "query": { "match_all": {} } }'

or, if you have the query stored in query.json

curl -XPOST -H 'Content-type: application/json' "http://localhost:9021/_search" -d@query.json

Here's a doc about how to switch search engines: https://fusionauth.io/docs/v1/tech/tutorials/switch-search-engines

You can experiment in the admin UI using the search field.

You can use the query string DSL or a full JSON query in the search field, if you are running elastic search.

Here's a link to building an ElasticSearch query (in version 6.3): https://www.elastic.co/guide/en/elasticsearch/reference/6.3/query-dsl-query-string-query.html

For example if you have companyName in custom data, the query string would be data.companyName:"Acme Corp."

If you are using the database search, you won't be able to use this kind of querying.

Here's more on the difference between the database search and elasticsearch options for user search: https://fusionauth.io/docs/v1/tech/core-concepts/users#user-search

You can search on the user id. While it doesn't say that in the placeholder, it works.

No, but you can flush the index: https://fusionauth.io/docs/v1/tech/apis/users#flush-the-search-engine . From that doc:

This will cause any cached data to be written to disk.