As far as I experienced you can do it - but more manually:
You have to set the group as default/ standard group and then you can invite people into.

As you see, this goes crazy if you have multiple invites to different groups. So it could be an option to pick the external users out of the generic group and sort them one by one into their specific group.

That has three problems:

• To invite external users, you must be site collection administrator, Full Control on target subsite is to less.

• You can handle external users inside SharePoint only after they accepted their invitation.

• And it requires that external users have something like read permission on root collection site to avoid the user experience "access denied".

My way to go is creating a website collection for each company/ project with the same target group and put them into the generic groups. Give them specific rights (for group) and/ or separate them after first contact.

As far as I experienced you can do it - but more manually:
You have to set the group as default/ standard group and then you can invite people into.

As you see, this goes crazy if you have multiple invites to different groups. So it could be an option to pick the external users out of the generic group and sort them one by one into their specific group. You need to keep an eye on which subsites should be visible to everybody in navigation bars.

That has three problems:

• To invite external users, you must be site collection administrator, Full Control on target subsite is to less.

• You can handle external users inside SharePoint only after they accepted their invitation.

• And it requires that external users have something like read permission on root collection site to avoid the user experience "access denied".

My way to go is creating a website collection for each company/ project with the same target group and put them into the generic groups. Give them specific rights (for group) and/ or separate them after first contact.