Programmatic way of sharing a 0365 site site with external users

Hello Everyone 

Is there a programmatic way of share a 0365 site with external users ?  

My requirement is that , We have 2 Tenants "Tenant A"  and "Tenant B"  , Now we need  all users of "Tenant A"  need to access sites of "Tenant B"  , But we don't need to add all users of "A" to "B "  as that need separate license for each user .So we need to share site of "B" with "A"  . We have more than 1000 users in each tenant so manually sharing for each users is not going to a feasible option . 

How do we solve this ? Is there any possible ways ? 

Thanks in Advance for your help and suggestions 

Regards Sooraj