can a user instal themelf a plugin

hi

can a user instal themself a plugin ?

and if so how?

why becas ther are plenty of nice wordpres plugins that i whant to give to my free blog users

can i install wordpres plugins only for my main site alone. and not the sub blogs.?

can i instal a wordpres plugin only for some blog users and some not?

I like to buy this plugin seopressor and give it to my users for free

the versi SEOPressor UNLIMITED say Multi-site License Unlmited Domains so i think it wil be olright when i give it to my subdomains users .