Hi Andrea,
Currently, BuddyPress only shows the members of the first profile field group on the registration page.

You can still achieve your goal by modifying the register.php.

Does your theme have buddypress/members/register.php file? If not, please copy plugin/buddypress/bp-templates/bp-legacy/buddypress/members/register.php to your theme/buddypress/members/register.php

Now, if you look into this file, you will see a line like this

	<?php if ( bp_is_active( 'xprofile' ) ) : if ( bp_has_profile( array( 'profile_group_id' => 1, 'fetch_field_data' => false ) ) ) : while ( bp_profile_groups() ) : bp_the_profile_group(); ?>

Please change that to

<?php if ( bp_is_active( 'xprofile' ) ) : if ( bp_has_profile( array( 'fetch_field_data' => false ) ) ) : while ( bp_profile_groups() ) : bp_the_profile_group(); ?>

We are effectively saying BuddyPress to load all groups on registration page.

After that, you can use any javascript wizard plugin to make it multi step registration.

PS: if you do not want to load all the profile field groups and want to show only specific, you will have to use the “exclude_groups” parameter(There is no include groups for multiple field groups) in the bp_has_profile() args.

Hope that helps.

Regards
Brajesh