A
   --------------------
1  alice@example.com
2  bob@example.com
3  charlie@example.com
4  dave@example.com

A                    B
   -------------------- ---------------------
1  alice@example.com    alice@example.com
2  bob@example.com
3  charlie@example.com
4  dave@example.com

A                    B
   -------------------- ---------------------
1  alice@example.com    alice@example.com
2  bob@example.com      alice@example.com;bob@example.com
3  charlie@example.com
4  dave@example.com

A                    B
   -------------------- ---------------------
1  alice@example.com    alice@example.com
2  bob@example.com      alice@example.com;bob@example.com
3  charlie@example.com  alice@example.com;bob@example.com;charlie@example.com
4  dave@example.com     alice@example.com;bob@example.com;charlie@example.com;dave@example.com

=CONCATENATE(TRANSPOSE(A2:A),", ")