A rightward facing triangle is available as a UTF-8 character, 9668.
To using the collapse manipulator from
collapse clade, you can then annotate the tip with the triangle as a geom_text2. You will need to change the hjust and vjust to adjust position and the cex to make it the right size. For example, if you wanted to collapse node 232 from the example bird.families dateset from ape:
library(ggtree)
library(ape)
data(bird.families)
coll <- ggtree(bird.families) + geom_tiplab(cex=1.2) + xlim(0,70) + ylim(0,130)
coll <- coll %>% collapse(node=232)
coll <- coll + geom_text2(aes(subset=(node == 232)), cex=2, label=intToUtf8(9668), hjust =.2,vjust=.45)
coll <- coll + geom_text2(aes(subset=(node == 232)), cex=1.5, vjust=0.2, label="Group",hjust = -.5)
coll
The collapse() manipulator also shortens the tree, which makes it impossible to represent how many taxa are included in the group. There would also be a way to collapse while preserving shape by grouping with OTUs, but that would get really complicated quite quickly. Easier to just do it manually if you have many collapsed nodes.
If you any other questions, don't forget to include some reproducible code by using the rtree() function, or the bird.families dataset.