Easily done using SQL Actions. The majority of the plugins can be or are done with just queries. We used integration plugins to make it simple. However, for in-depth usage of a component I would suggest using queries. Yes, you can do this for groupjive with a query as SQL Actions supports substitutions.

Here are example queries, both will need to be ran. One creates the group. The other inserts the user into the group.

[code:1]
INSERT INTO `#__gj_groups` ( `name`, `descr`, `date_s`, `type`, `creator`, `user_id`, `active`, `category` ) VALUES ( 'My Group', 'My Group Description', NOW(), 1, '[username]', [user_id], 1, [cb_gjcategoryID] );
INSERT INTO `#__gj_users` ( `id_user`, `id_group`, `date`, `status` ) VALUES ( [user_id], '', NOW(), 'active' );
[/code:1]

Complex SQL is going to be needed to grab some IDs. Such as groupid and categoryid. Using a standard drop-down field from field management will not return a categories ID, but its name instead. So you need to do a select query inside the insert query to grab the ID.

Example:
[code:1]
INSERT INTO `#__gj_groups` ( `name`, `descr`, `date_s`, `type`, `creator`, `user_id`, `active`, `category` ) VALUES ( 'My Group - [user_id]', 'My Group Description', NOW(), 1, '[username]', [user_id], 1, ( SELECT `id` FROM `#__gj_grcategory` WHERE `catname` = '[cb_gjcategory]' ) );
INSERT INTO `#__gj_users` ( `id_user`, `id_group`, `date`, `status` ) VALUES ( [user_id], ( SELECT `id` FROM `#__gj_groups` WHERE `name` = 'My Group - [user_id]' ), NOW(), 'active' );
[/code:1]

As you can see I've done a select within an insert to get category ID and groupid. I've also changed the group name to be unique in order to grab its ID from database.

This is just an example! Ensure you make any necessary modifications and run any necessary testing. This has NOT been tested.