all is or all are

Hi,

I think the word 'all' can be followed with (should I use 'followed by'??) a singular or plural verb depending on the situation.

He will guide all who follow Him.   

I know you have four members in your group. All is/are?? welcomed.

I'd say 'all' can only be followed by a singular verb when the meaning of 'all' is 'everything' -- and that is probably limited to well-known expressions most of the time:

All is (not) lost.
All is forgiven.
All is fair in love and war.
All's well that ends well.
All is calm, all is bright.