You Do Need power plug adapter while traveling from Guam to Montenegro.

Here is Why You Need An Adapter

The outlet and plug type used in Montenegro are C and F and in Guam are A and B. This is how they look like.

Guam

In Guam, outlets of type A and B are in use.

Type A

Type B

Montenegro

In Montenegro, outlets of type C and F are in use.

Type C

Type F